Theresa May has promised a "Great Repeal Bill" to transfer EU law to the UK.

Thousands of pieces of legislation will need to be moved, amended or scrapped before Britain leaves the EU.

Government to be handed powers to remove EU laws at the swipe of a pen.

House of Lords warn of a "massive transfer of legislative competence from Parliament to Government."

Campaign groups fear hard-won rights will be lost.

LONDON — The Great Repeal Bill has been sold as a means of "taking back control" of UK laws from the EU. In reality, it risks being the single biggest power grab by a British government in modern times.

Here's the big problem:

Repealing EU law is a minefield.

The prime minister announced last year that she would effectively remove Britain from all EU law. She said she would do this through a "Great Repeal Bill" which would "remove from the statute book – once and for all – the European Communities Act."

This would ensure that "our laws will be made not in Brussels but in Westminster. The judges interpreting those laws will sit not in Luxembourg but in courts in this country. The authority of EU law in Britain will end."

Of course, it's not quite as easy as that. Were the prime minister to simply scrap all EU laws, Britain would grind to a halt. So, in the interim period, the PM plans to take a "snap shot" of all existing EU laws and transfer them over to UK law where they can later be amended or scrapped at will.

It sounds brilliantly simple but in reality, it's a nightmare. Here's why:

There's too much of it.

Getting rid of EU law isn't simply a case of repealing one act and cutting and pasting it over into UK law. According to the EU, there are around 20,000 EU legislative acts in force, of which 5,000 apply to all member states. And while some of it's effective by virtue of the ECA, much of it isn't.

Huge parts of EU law are already embodied in both primary and secondary UK legislation, while other parts are not really laws at all but judgments made by the Court of Justice, or rulings by EU regulators. Deciding which parts of this will need to adopted or amended is a bureaucratic nightmare that would take many years to do properly. Britain has about 18 months.

As Daniel Greenberg, a former Parliamentary council and expert on legislative law, told Business Insider, reviewing all that EU law to see what should be transferred, ditched or amended will be "a civil service legal exercise on a scale that has not been encountered at any other time in our recent legal history."

It's still changing.

The other problem is that we won't know what laws we want to keep until the Brexit negotiations are over.

May has promised to secure a free-trade deal with the EU, but in order to do so, she will have to accept the continuation of a certain amount of EU laws and regulations, particularly in those areas that govern trade. Quite how much she will have to accept depends on negotiation, which almost certainly won't conclude until the last minute. This creates another problem.

REUTERS/Toby Melville

We don't have time.

Unpicking thousands of pieces of legislation and asking Parliament to vote on which bits to keep and which bits to scrap would take decades, several governments and would almost certainly be impossible.

Because of this, the government has another plan and here's where the power grab comes in.

Delegated powers

There's only really one solution to May's difficulties and that is to bypass Parliament altogether. This can be done through the use of delegated powers and statutory instruments. These allow the government to amend laws without first facing parliamentary scrutiny. These powers were intended to be used by governments only for minor time-sensitive technical changes to laws. However, the time pressures caused by May's Brexit timetable means that they will almost certainly have to be used much more widely and liberally.

This is extremely dangerous.

As the House of Lords Constitution committee warns today in a new report, the Great Repeal Bill will "involve a massive transfer of legislative competence from Parliament to Government." They add that "this raises constitutional concerns of a fundamental nature, concerning as it does the appropriate balance of power between the legislature and executive."

The Great Repeal Bill will hand ministers what human rights organisation Liberty describe as "virtually untrammelled power" to make and unmake whole swathes of our law. Everything from workers rights' to food standards, to banking regulations, will suddenly be up for grabs by Downing Street and it will all happen so quickly that we won't even realise what we've lost until it's gone.

There are no safeguards.

The government have insisted that nothing of importance will be lost. On workers' rights, in particular, they have been keen to dismiss warnings from Labour that citizens' hard-won rights will be lost. The Brexit secretary David Davis has also promised that any major changes will go through primary legislation and be voted on by Parliament. Maybe it will.

Maybe it will. Maybe the government will stand back and refuse to use the sweeping new powers that the Great Repeal Bill affords them.

But the problem is that so far the government have only offered warm words on the subject. There have been no announcements of new safeguards to prevent the government from simply rewriting Britain's legal settlement at the swipe of a pen. Calls from the Lords for the government to promise to insert so-called "sunset clauses" to make the changes to laws temporary have so far not been met. Calls for a new scrutiny body in parliament to check any changes have also so far fallen on deaf ears.

Time is running out for the government to spell out how they plan to use the powers soon to be afforded to them. But as things stand there is absolutely nothing in place to prevent the biggest political power grab of modern times.