Image copyright Supreme Court Image caption The Supreme Court should be the ultimate legal authority in the UK, the MP says

The European Court of Justice's involvement in the UK should end on "day one" of Brexit, Conservative MP Jacob Rees-Mogg has said.

The prominent backbencher told the BBC if the court's jurisdiction continued, even for an interim period, the UK would not really be leaving the EU.

The UK is currently at odds with the EU over the court's role in enforcing the rights of EU citizens after Brexit.

He also said the implementation phase for Brexit should be taking place now.

Rather than waiting until the UK's official departure in March 2019 to begin the "transition" to post-Brexit rules, departments should be implementing the changes where they could during the period up to the March 2019.

The role of the European Court of Justice - the ultimate legal arbiter of disputes between member states and the EU and cases brought by companies and citizens under EU law - is one of the most controversial aspects of the Brexit process.

The UK government has said the rights of EU citizens who remain in the UK after Brexit will be enshrined in domestic law and must be enforced by British courts in future. The EU has said the European Court of Justice must continue to have some form of oversight role.

But Mr Rees-Mogg said he regarded the issue as a red line for the UK.

"It (the European Court of Justice) cannot be our senior court for a day after we leave the EU," he told BBC Radio 4's Today. "It is about control. Do we make our laws according to our own democratic principles on the day we have left or not?

"If we don't, we are still in the European Union. That is the essence of the whole debate... the court must just stop on the day we have gone."

For instance, he said that if the UK wanted to introduce a system of criminal checks for EU nationals entering the country, then it should be able to do so without facing a challenge in the European Court of Justice.

What does the European Court of Justice do?

It decides whether the institutions of the EU are acting legally, and it settles disputes between them.

It ensures that the member states of the EU are complying with their legal obligations as set out in the EU treaties; and it allows member states to challenge EU legislation.

It interprets EU law at the request of national courts.

Taken all together, this means that the ECJ interprets and enforces the rules of the single market, and pretty much everything else that the EU does.

Reality check: What is the European Court of Justice?

By leaving the EU and its single market and customs union, the UK would no longer be subject to European Court of Justice rules but some MPs have floated the option of remaining in the European Economic Area for a transitional period, which would mean still being overseen by the European Court of Justice.

A debate has been going on in Theresa May's cabinet about the terms of any Brexit "transition" period and what it will mean in terms of access to markets and labour as well as continued financial obligations.

Ministers have said the so-called "implementation" (or transition) phase - in which aspects of EU membership could be temporarily retained to minimise disruption to business - could last until the next election in May 2022 but Downing Street has insisted freedom of movement will end.

While there were issues that could only be dealt with after Brexit, Mr Rees-Mogg said there was a lot that Whitehall should be doing now.

"The implementation period should begin now. People should be getting ready now. Government departments should be getting ready now. They know what is going to happen. They know we are leaving."

On immigration, Mr Rees-Mogg said the process of gaining control of the UK's borders through Brexit was separate from attempts to reduce the numbers of people coming to the UK, which he suggested might take longer to happen.

"Once we have left and we have control, there is a political argument about what levels of immigration we should have... that is really going to be a challenge and an issue for the competence of the Home Office.

"It is not so much a question of Brexit. It is Brexit which makes it possible, it is then administrative efficiency that will make it happen."

The Conservatives have been committed since 2009 to reducing levels of net migration to the tens of thousands but have so far failed to meet the target and have faced calls from business and some MPs to scrap the target.