What is the European Court of Justice and why does it matter?

Britain's relationship to the European Court of Justice (ECJ) is in the news. But what is the ECJ and what does it do? We answer some of the key questions.

What is the European Court of Justice?

Located in Luxembourg, the ECJ oversees the correct application and implementation of EU law. It consists of judges from all member states. The UK has three judges, one in the Court of Justice and two in the General Court - the lower level court to the ECJ.

The UK also has an advocate general in the court. An advocate general gives legal opinion on important cases which may be influential but is not binding on the court.

What sorts of cases does the ECJ rule on?

The ECJ can hear cases from the national courts through the 'preliminary ruling' system. This involves a national court referring a question on the interpretation of EU law to the ECJ. The ECJ decides the correct interpretation and sends the case back to the national court for a final decision. It is still up to the national court to decide issues of its own nation's laws.

The European Commission can also take a case against an EU state to the General Court. These cases ask the court to decide whether the member state is in breach of its obligations to the EU. In some narrow cases, such as a state being a repeat offender and where it has failed to implement an EU directive, the court can fine a member state.

An EU state can also take another member to the court. This does not happen often, only where there is a political interest for the state to ask the court about a point of EU law.

Finally, the court can perform a judicial review of EU law. This means it has the power to review EU regulations and directives and make sure they comply with EU treaties and general principles of law.

Why is it such a political hot potato?

The court has powers over EU member states, including deciding if the UK government has breached EU law. Some people feel it is not right for a non-UK based court to have such power.

In the EU system, EU law applies directly in each country and by being a member of the EU we have agreed to let it override our national law. For example, if a member state passed a law that imposes restrictions on recognition of qualifications from other EU states, an individual or business could challenge these restrictions in the national courts. If there are serious consequences, they could ask their local court for compensation from their government. Their local court would then have to apply EU law.

UK membership of the ECJ surfaced repeatedly in the run-up to the Brexit referendum and has continued to be a prominent topic. For example, Michel Barnier, the EU's chief negotiator, said the ECJ should have the power to fine the EU or the UK if either side is in breach of a future agreement on citizens' rights or the divorce bill flowing from Brexit.

Some see leaving the jurisdiction of the ECJ as an important symbol of taking back sovereignty over our laws, while others see staying within the ECJ's jurisdiction as vital if we want to continue to have access to key EU organisations, which bring big benefits to our businesses, over which the ECJ has oversight.

So what would be the impact of leaving the ECJ?

The UK courts could no longer take cases to the ECJ if there were arguments over how to interpret EU law. The European Commission could not take infringement action against the UK. The supremacy and direct effect of EU law in the national courts would end.

However, whether we could ever completely escape the ECJ's reach is less certain. Former ECJ judge Professor Sir David Edward made the point that: ‘You can escape the jurisdiction of the ECJ, but you have got to comply with EU standards if you are going to export into the EU.’ Who decides the standards? Well, ultimately they're interpreted by the ECJ.

Also, if we leave the ECJ's jurisdiction there will no longer be UK judges at the court. So it will influence us - but we will not be able to influence it.

Are there benefits to staying?

While we remain in the ECJ we can have UK judges appointed to it and it allows our citizens and businesses to take cases when their rights or obligations under EU law are not being upheld.

Give me an example of a recent case where the ECJ ruled on a UK issue

Concerning the clean air directive the UK was found to be in breach of its EU law obligations in ensuring clean air

But the UK is not alone here. The European Commission has brought actions against 16 member states on this legislation.