Parliamentarians will debate the EU Withdrawal Bill – or the Great Repeal Bill as it’s known to its friends – for the first time today.

As bills go, it’s pretty short – just 66 pages. But it’s a feisty little fella’, and is already being described as the government’s greatest Brexit challenge.

FactCheck takes a look at what it means for the government.

What is the Great Repeal Bill and why do we need it?

To understand the Great Repeal Bill we need to understand what it is repealing: the European Communities Act (ECA), which was passed in 1971 by Prime Minister Edward Heath. It took effect the following year.

The ECA was, in its own right, a landmark piece of legislation. It has given successive governments the ability to “transpose” EU directives and regulations into domestic UK law using secondary, rather than primary, legislation.

Why is that significant?

Well, primary legislation (made through Acts of Parliament) can take up to a year to pass, requires debate and scrutiny in both the Commons and the Lords, and can be amended by parliament and parliamentary committees.

Secondary legislation, on the other hand, is much more straightforward: it takes between three weeks and three months to pass, is often exempt from debates, and can’t be amended by parliament.

The ECA has allowed EU laws to find their way onto the British statute with relative ease, freeing up precious parliamentary time for governments’ domestic legislation.

The Great Repeal Bill – if and when it passes – will overturn the ECA. The government will no longer have the power to pass EU legislation into UK law, which makes sense, given we’re leaving the trade bloc.

But that’s not the only thing that the Repeal Bill will do. It will also copy and paste all the EU laws that the UK currently observes into our own statute book. At first glance, that’s a bit counterintuitive: if we’re leaving the EU, why should EU regulations continue to bind us in domestic law?

The government says that the point of “converting and preserving” EU law into UK law is to “maximise certainty for individuals and businesses as we leave the EU in order to provide the basis for a smooth and orderly exit”. They say that “it is in no one’s interests for there to be a cliff edge when we leave the EU, and so the laws and rules that we have now will, so far as possible, continue to apply.”

The Great Repeal Bill is “the greatest constitutional change since the war”

But, as the government knows, “continuing and preserving” EU law is not as straightforward as it sounds. Hence, the third prong of the Repeal Bill: it gives the government power to “correct” bits of EU law that don’t quite work when they’re copied and pasted into UK statute.

Crucially, the correcting power allows ministers to make such changes using secondary legislation – i.e. without much scrutiny from parliament. This is sometimes called a “Henry VIII power”, after the Tudor king passed a law that allowed him to legislate by proclamation in 1539.

Now, the government says that the correcting power must only be used to fix technical snags, rather than making substantive changes to policy. They say “it is not a power to change laws because the government did not like them before exit.”

That’s all well and good, until we look at the language of the Repeal Bill itself. The key passage is section 7(1), which says:

A Minister of the Crown may by regulations make such provision as the

Minister considers appropriate to prevent, remedy or mitigate—

(a) any failure of retained EU law to operate effectively, or

(b) any other deficiency in retained EU law,

arising from the withdrawal of the United Kingdom from the EU

Put simply, a government minister can make any secondary legislation they want if they believe it necessary to correct virtually any problem – or perceived problem – with EU law.

A power this broad is, according to constitutional experts, highly unusual.

We spoke to Vernon Bogdanor, CBE, Professor of Government at King’s College London. He said the Great Repeal Bill is “the greatest constitutional change since the war”.

And he predicts that there will be two main issues in the committee stage (which is when MPs and peers scrutinise and propose amendments to the bill).

The first is “the fact that this is being done by executive [government] action – so-called Henry VIII clauses with minimal parliamentary scrutiny”.

He says “the government’s argument for this is that it is merely transposing law. But is that right? What, for example, is the British equivalent of the European Commission? Surely, political choices are involved. Therefore, the opposition argue, there should be better parliamentary scrutiny.”

The second problem he identifies is “devolution issues”. He points out that policies like agriculture and fisheries were given to the devolved administrations in devolution legislation: “in theory, therefore [control of those policies] should go directly to the devolved bodies”. But he says that “the UK government argues that one cannot have four widely different systems of agricultural subsidy in the UK [so they are] keeping all functions at Westminster for the time being”.

As Professor Bogdanor says, “naturally the devolved bodies are annoyed about this”.

So it looks like the government could be on a collision course with the Westminster parliament for not giving it enough opportunity to scrutinise legislation, and with Edinburgh, Cardiff and Belfast for apparently changing the terms of devolved policy.

Could all this set a precedent for future power grabs?

Professor Bogdanor thinks not – precisely because the Great Repeal Bill is so unusual: “arguably, therefore, it does not set a precedent for future legislation. It is very much a one off”.