Although we haven’t yet much clarity about what Brexit is going to look like in practice, we are starting to get some more details about the process. The Prime Minister has told us that she intends to trigger the article 50 process of leaving the EU at the end of March 2017. And while her plans may be derailed if the Courts decide that Government has no power to trigger article 50 without Parliamentary approval, this means that if all goes to plan the UK would leave the EU on April Fool’s Day 2019.

The PM has also announced a “Great Repeal Bill” that would “remove the European Communities Act from the statute book. … It will return power and authority to the elected institutions of our country. It means the authority of EU law in Britain will end.” We are told that the Bill will be introduced in the next session of Parliament, which begins in May. But how will the Bill be passed and what will it mean for the UK once enacted?

The first thing to note is that, like other Bills, and unlike the Government’s preferred process for article 50, the Great Repeal Bill will have to be approved by Parliament. There will need to be votes from both Houses of Parliament in favour of both the principles and the details of the Bill. As the Government has a majority of only 16 seats in the Commons, no majority in the Lords, and many in both Houses have expressed a desire to slow or even stop the process of Brexit, securing these votes cannot be taken for granted. While MPs and Peers might be slow to challenge the principle of leaving the EU, given the result of the referendum, they might be more willing to challenge the details of any particular form of Brexit, as no such details were accurately described during the referendum campaign. So the Great Repeal Bill is unlikely to contain many details, reducing the scope for Parliamentary defeats.

This also helps with the second issue, which is that EU law has had effect in the UK for over 40 years and no doubt some of this will have to remain in force, even after Brexit. Some EU laws give effect to international treaty obligations which will continue to bind the UK, some set the standards for goods and other products that UK producers will need to meet if they are to sell into the EU, and some simply have no British domestic equivalents. All of these will need to stay in place, albeit converted into UK laws, to avoid a legal vacuum. Some other EU laws will remain in place as part of the Brexit agreements if the UK agrees to remain part of EU-wide organisations, for instance such as the European Medicines Agency. As the public bill committee in the Commons gives members of the public the chance to give evidence about the scope of legislation, this could be a chance for outside groups to seek to protect EU laws of particular interest to them. More cynically, some EU laws are seen by civil servants as a useful constraint on their Ministers, and so they will be seeking to keep them in place after Brexit. But none of this is known in detail yet – and much may not be known even after the Brexit negotiations conclude – so very few details will be able to be included in the Bill.

Third, while the UK remains a member of the EU, we are bound by its rules, and the PM has regularly stated the Government’s intention for the UK to “be a fully engaged and active member of the EU until the point at which we leave.” Repealing EU laws now would put the UK in breach of those rules, so the Great Reform Bill will not have any immediate effect on EU law, which will continue to bind the UK until that point.

So the Bill is likely to be brief: clause 1 of the Great Repeal Bill will no doubt repeal the European Communities Act 1972 (which provides for EU law to have effect in the UK, and gives the European Court of Justice its primacy) from the date of Brexit. The Bill will then provide for all EU laws in effect before that date to continue in force until repealed or replaced, as purely domestic law. Because of the amount of Parliamentary time that it would take to pass a new Act for each current EU law, the Bill is also likely to include a wide-ranging power for Ministers to repeal or replace those laws. This will be an extremely wide-ranging “Henry VIII” power to change primary legislation by order. Ironically enough, the most likely precedent is the power in the European Communities Act for Ministers to amend UK laws to give effect to EU obligations. However, this power is likely to be even wider, both to repatriate enforcement mechanisms (as EU bodies won’t be enforcing the UK’s version of its regulations), and to make future changes. Parliament is likely to want to impose a number of safeguards on the exercise of this power. Devolved governments are also likely to want to see safeguards, as a lot of EU law relates to devolved matters, such as agriculture or the environment, and so the Bill may have to explain how these will be dealt with too.  Both Parliament and the devolved bodies may therefore find that a vote to “take back control” has resulted in control moving from them to Whitehall.

Nicholas Evans is a partner and parliamentary agent in the Government and Infrastructure Department at Bircham Dyson Bell.