The European Court of Justice is the guardian of these Directives and Regulations adopted by the Council. It makes decisions about whether individual States or Institutions are in breach of EU Law. The European Commission consists of 20 members chosen from various countries to work as civil servants who serve the community. It proposes EU policies and presents draft legislation to the Council.
The European Parliament is made up of elected representatives from constituencies across the European Union for a fixed five-year term; it’s purpose is to advise the Council of Ministers on commission proposals.
In the late 20th and early 21st Century, the constitutional principle of Parliamentary supremacy underwent erosion. The erosion has been connected with Institutions of the European Union, in particular the European Court of Justice that asserts the power to exercise judicial review over the UK law. According to the court, every member state that joined the Community has transferred legislative sovereignty to the Community. In a situation where any adverse finding by the Court that UK law is inconsistent with the agreed Treaties, it automatically annuls the law, since the European Communities Act provides that European Union law is supreme in the United Kingdom. This was made very clear in Costa v Enel  ECR 585. This undermines the power of Parliamentary sovereignty, s2(4) of the ECA does enable UK courts to give priority to EC law as the ECJ requires. If there is a clash between UK domestic law and EU law, the EU law will take precedence therefore Parliament is no longer sovereign. This can be illustrated by the Factortame case (1990) the ECJ held that the English courts could not apply the Merchant Shipping Act (1988) designed to protect British fishermen because it conflicted with the Treaty of Rome 1957. However, the UK can withdraw for the EU at anytime it wishes.
EU law is directly applicable as soon as the law is created in the EU; it becomes enforceable to all the EC countries by individual citizens without there being any domestic legislation. Treaties, regulations and decisions of the ECJ are all directly applicable. This undermines Parliaments power because it is no longer the sole supreme lawmaker of the UK. However, directives are not directly applicable meaning that a citizen of the EU cannot use the legislative until the country has brought the legislation into force. The directive needs to be enacted by Parliament thus giving law-making powers back to Parliament.
It can be argued that Parliament is progressively losing power, an example of this is the devolution of power to regional assemblies in Scotland (Scottish Parliament), Wales (Welsh Assembly) and Northern Ireland (Northern Ireland Assembly). This is yet another example of Parliament losing power although not in Europe. Parliament still remains dominant in England.
Practical reality dictates that the British Government is the majority party in the House of Commons. Strong party dicipline makes this majority reliable and almost guarantees the Government victory in Commons votes- an elective dictatorship. Also backed by the Civil Service “machine” it is easy to argue that sovereignty actually lies with the executive not Parliament. This is another example of Parliament losing power.However, Prime Ministers who systematically repress the powers of party and Parliament tend to meet their fate – Margaret Thatcher is a classic example of this.
At least once every five years the House of Commons is re-elected, and so at that point sovereignty really lies with the people. However, after the general election sovereignty returns to Parliament for the next five years.
Parliamentary Sovereignty is being undermined by the increasing use of referendums, which take away power from Parliament, as in reality, the decision whether to pass law is made by the electorate, not Parliament. However, Parliament could still reverse decisions made by referendums.
Parliament still remains dominant with the UK and will continue to do so because no Parliament can be bound by its predecessor hence no Parliament can bind its successor. One Act expressly repeals an earlier one if it explicitly says that the earlier act is repealed. Acts can also imply that other Acts have been repealed, if an Act of Parliament says something which conflicts with an earlier Act, the later Act will be followed. This is a principal of Parliamentary sovereignty that an Act of Parliament can repeal any earlier one. This point demonstrates how Parliamentary sovereignty works out side Europe and that it is the keystone of the UK constitution.
There are examples of some Acts that can only be repealed if the statutes are overrules expressly, these were called constitutional statues, an example of this is the Act of Union 1707.
The dominance of the EC within the UK can be described as a mere illusion because it is limited to certain fields such as trade and agriculture, the EU is not concerned with other aspects of UK law such as family and health. Parliament remains sovereign in more domestic areas.
Parliament may abolish any of the devolved legislatives at it pleasure. The European and British courts have the authority to declare incompatibility or to annul a law only because of an Act of Parliament. Thus, theoretically, Parliament remains almost entirely sovereign.There is a concept in political science of legal and political sovereignty. It can be argued that legal sovereignty has not been lost, because Parliament still retains all its theoretical powers. However, as it is highly unlikely that the UK would repeal the European Communities Act and leave the European Union.