The WAB turns Boris Johnson's withdrawal agreement, which is a draft international treaty, into UK law and gives the government permission to ratify it.

Bills to implement major European treaties usually take several weeks to get through Parliament, and sometimes considerably longer.

But the government is determined to push this bill through pretty quickly, to avoid another Brexit delay.

The opposition would have liked more time to scrutinise the bill in detail. It is, they argue, one of the most important pieces of legislation in decades, and MPs need to be sure they know exactly what it says and does.

But the government insists that sufficient time for scrutiny will be given.

So what does the WAB actually cover? Among other things:

It sets out exactly how the UK will make "divorce bill" payments to the EU for years to come

It repeals the European Communities Act, which took the UK into the EU, but then reinstates it immediately until the end of 2020 when the transition period ends

It contains language on how the new protocol on Ireland - setting up what amounts to a customs and regulatory border between Northern Ireland and Great Britain - will work in practice

It sets out areas in which the European Court of Justice still plays a role in the UK, and makes the withdrawal agreement in some respects "supreme" over other areas of UK law

One of those areas may be in the arbitration procedure for disputes about the withdrawal agreement. The bill introduces a duty for the government to report on this

It prohibits any extension to the transition period beyond the end of 2020, even if a free trade deal isn't ready in time. The previous version of the bill allowed for the possibility of an extension

In the section on citizens' rights it sets up an independent monitoring authority (IMA) with which EU nationals in the UK can lodge any complaints about the way the government treats them

In several policy areas, particularly in Northern Ireland, the bill gives ministers a lot of power to change the law (through secondary legislation) without MPs getting to vote

It introduces a duty for the government to report on its use of the arbitration procedure for disputes about the withdrawal agreement

The previous version of this bill included a clause on workers' rights, but the government has removed it and says it will now be part of a separate bill.

After the WAB becomes law, the withdrawal agreement also needs to be ratified by the European Parliament next month.

Then the stage will be set for Brexit on 31 January, when the post-Brexit transition period will begin.

For 11 months, the UK will still follow all the EU's rules and regulations, it will remain in the single market and the customs union, and the free movement of people will continue.

The challenge for the government will be to get all its new rules and policies in place by the end of next year.