Parliament-the supreme law-making body-may legislate on any subject matter.
No parliament can be restricted by a predecessor or restrict the power of a future Parliament.
No body, including a court of law, may question the validity of Acts of Parliament.
Though parliamentary sovereignty was proclaimed in such wide terms, it has to be accepted that Parliament has its constraints in fully exercising the amplitude of its powers. In fact disincentives for exercise of unauthorised and oppressive power could be found in constitutionalism and rule of law.
A recent literature on the topic suggests that the following can be identified as the core elements of constitutionalism: 
i) the recognition and protection of fundamental rights and freedoms,
ii) the separation of powers,
iii) an independent judiciary,
iv) the review of the constitutionality of laws, and
v) the control of the amendment of the constitution.
On account of the existence of conventions there is also a presumption that parliament cannot use its powers in an unauthorised manner. In spite of the lack of written constitutional limits on the United Kingdom Parliament,  it has long been recognized that there are certain measures that it would be politically impossible to adopt and whose enactment would, therefore, never be attempted. As the Lord Chancellor, Lord Sankey, explained in British Coal Corporation v. The King :
It is doubtless true that the power of the Imperial Parliament  to pass on its own initiative any legislation it thought fit extending to Canada remains in theory unimpaired: indeed the Imperial Parliament could, as a matter of abstract law, repeal or disregard section 4 of the Statute [of Westminster]. But that is theory and has no relation to realities. 
I would say that the current constitutional landscape in England is causing the net around parliamentary sovereignty to tighten: the category of legislation that it is politically impossible for Parliament to enact. Such developments alter radically our approach to the theory of parliamentary sovereignty so that one wonders whether there is an omnipresent Parliament in today’s context. I would discuss them under the following rubrics.
Parliamentary sovereignty and the European Union
The United Kingdom has been a member of the European Union since 1973. Long before then, the European Court of Justice had explained that EU law took priority over the laws of individual states.  The Court stated that “the Member States have limited their sovereign rights, albeit it within limited fields, and have thus created a body of law that binds both their nationals and themselves,"  articulating a comprehensive principle of EU law supremacy that bites even on domestic constitutional laws.  The rationale for this principle is self-evident: the raison d'etre of EU law is to create (within certain fields) a pan-European system of regulation and body of rights, something that would be unattainable were member states able to opt out simply by adopting contrary domestic provisions. The difficulty, though, is that if EU law is supreme, then this appears to conflict with the orthodox position vis-à-vis parliamentary sovereignty.
Although the principle that EU law takes priority over domestic provisions was firmly established early in the life of the Union, relatively little attention was paid to this issue when the United Kingdom joined. The British legislation that was enacted to give effect to EC accession only stated that “any enactment, passed or to be passed, … shall be construed and have effect subject to" directly effective  EU laws. Surprisingly, when the Lords took the groundbreaking step of issuing an injunction to suspend the operation of an Act of Parliament Factortame case, the court said little about the implications for the doctrine of parliamentary sovereignty. Only one judge addressed this issue in any meaningful way, and even then confined himself to remarking that the supremacy of EU law was “well established … long before the United Kingdom joined the [European Union]. Thus, whatever limitation of its sovereignty Parliament accepted when it enacted the European Communities Act 1972 was entirely voluntary." 
Britain's membership in the EU therefore demonstrates the inadequacy both of sovereignty theory's absolutism, given that Parliament today is clearly constrained by EU law..
Parliamentary sovereignty, human rights and devolution
The difficulties that Britain's membership in the EU poses for the traditional version of parliamentary sovereignty are mirrored, in a less developed form, by recent major reforms to the U.K. Constitution. 
The Human Rights Act  gives effect in U.K. law to many of the provisions of the European Convention on Human Rights (ECHR). None of the rights contained in the ECHR are placed beyond parliamentary interference by the HRA: indeed, when the government set out its proposals for legislation in this area, it emphasized its ongoing attachment to the traditional doctrine of parliamentary sovereignty.  This, however, evidences a rather myopic view, which unhelpfully dislocates legislative power from the wider political environment within which it subsists, and which the HRA has changed radically. The attention of Parliament is now systematically drawn to the human rights implications of draft legislation: its enactments must, whenever possible, be read consistently with relevant provisions of the ECHR--an obligation that the courts have discharged with notable enthusiasm in some cases. Further, some national courts4 are empowered to issue declarations of incompatibility if legislation is found to fall short of ECHR norms, thereby triggering the possibility of fast-track amendment by means of administrative legislation. In the unlikely event that these national provisions prove insufficient to secure respect for human rights in a particular case, there remains the prospect of proceedings before the European Court of Human Rights. As Lord Borrie said in a House of Lords debate on the Human Rights Bill (as it then was known):
the political reality will be that, while historically the courts have sought to carry out the will of Parliament, in the field of human rights Parliament will carry out the will of the courts … [T]he intention of the Bill surely is that government and Parliament will faithfully implement any declaratory judgment made by the High Court.
Like its power to depart from EU law, Parliament's ability to derogate from the ECHR, although formally undisturbed by the HRA, begins to look increasingly notional.51 A new political environment is emerging in which a legal doctrine of legislative supremacy appears at least anomalous.
The same point can be made in relation to the program of asymmetric devolution by which varying amounts of legislative and administrative power have been transferred to Northern Ireland, Wales and Scotland, reflecting different levels of public support for self-government in the constituent nations of the United Kingdom. Notwithstanding substantial differences among the schemes, an important common factor is that the U.K. Parliament has not renounced legislative sovereignty in relation to the three nations concerned. For example, the Scottish Parliament is empowered to enact primary legislation on all matters, save those in relation to which competence is explicitly denied, but this power to legislate on what may be termed “devolved matters" is concurrent with the Westminster Parliament's general power to legislate for Scotland on any matter at all, including devolved matters. In theory, therefore, Westminster may legislate on Scottish devolved matters whenever it chooses; in practice, however, it does not.
A constitutional convention rapidly emerged to the effect that “the U.K. Parliament would not normally legislate with regard to devolved matters except with the agreement of the devolved legislature." The reason for this convention is self-evident: unilateral interference in devolved matters by the U.K. Parliament would fundamentally undermine the spirit of the devolution scheme. It would be politically unacceptable for the Westminster Parliament to ignore the wishes of the Scottish people as expressed by their elected representatives in the Scottish Parliament. As time passes, and as devolution is woven ever more closely into the constitutional fabric of the United Kingdom, the theoretical ability of the U.K. Parliament to interfere unilaterally with devolved matters will be seen increasingly as a vestige of an unreconstructed doctrine of absolute legislative authority.
It is clear, therefore, that the doctrine of parliamentary sovereignty, in its traditional form, is increasingly under pressure. This phenomenon is demonstrated most clearly by Britain's membership in the European Union. In that context, as we have seen, EU law now enjoys de facto supremacy in the U.K., and the judiciary is joining the academy in contemplating re-evaluation of the doctrine of parliamentary sovereignty itself. In this way, theory may eventually come to reflect the contemporary reality that the United Kingdom Parliament no longer wields absolute power, but instead operates within a pan-European constitutional framework whose supremacy is the price of participation in the European Union. These collisions between traditional theory and contemporary political reality are not, however, confined to the implications of EU membership. Parliamentary sovereignty cannot consist as an island, untouched by the radical changes entailed in embracing human rights and dispersing governmental power among devolved legislatures. It is to be hoped that theory will ultimately catch up with these changes, although the history of the British Constitution suggests that the process will be a slow and gradual one.
The Sri Lankan cases too demonstrate the limits of legislative power. If a power could be classified as judicial, as distinguished from executive or administrative, such power could not be exercised by anyone other than a judicial officer. In Queen v Liyanage  the Supreme Court held that a provision conferring on the Minister of Justice the power to nominate the judges who would hear the case amounted to an interference with judicial power because the power of nomination had been traditionally exercised by the Chief Justice In fact the judges nominated in this case had declined to exercise jurisdiction in the case. The Supreme Court quite succinctly stated : “The fact that the power of nomination so conferred is capable of abuse so as to deprive a Judge of the entrenched power vested in him….or at least to derogate from that power, is a consideration which is not an unimportant one in deciding whether the conferment of this power……on a person who is not a Judge of the Supreme Court is ultra vires the Constitution,"  In Liyanage v The Queen  the Privy Council held that the Criminal Law (Special Provisions) Act (No 1 of 1962) and the Criminal Law Act (No 31 of 1962) usurped and infringed the power of the judicature. They noted that in passing the Acts, Parliament had no general intention of legislating either by the creation of crimes and penalties or by enacting rules relating to evidence, but that the Acts were clearly aimed at particular individuals who had been named in a White Paper and who were in prison waiting their fate. The alterations in the prevailing aw were limited to participants in a particular alleged coup.
The Privy Council further held that the Acts were a legislative plan ex post facto to secure the conviction of particular individuals and amounted to an exercise of judicial power by the legislature, which power could only be exercised by the legislature. These enactments were ad hominem legislative judgements. This judgement demonstrated a stark reality .i.e though the independence Constitution did not in express words confer a power of judicial review of the constitutionality of legislation on the courts, the courts exercised that power on the ground that it was implicit in the Constitution. One could see constitutionalism in its full glow in this instance This case underscores that there cannot be unlimited power vested in Parliament..
Thus I would conclude that the concept of parliamentary sovereignty is tempered by a host of factors enumerated above and Geoffrey Marshall’s statement holds true even today in an expanded notion.