The Brexit withdrawal agreement -which Theresa May and then Boris Johnson spent months desperately trying to get through Parliament in a variety of forms -has finally passed its third reading in the House of Commons, paving the way for Britain to leave the European Union on Jan. 31.

After a crushing general election victory for Johnson in December, the legislation sailed through with 330 votes in favour to 231 against, a majority of 99.

The bill still needs to clear the House of Lords, where peers can suggest amendments to the legislation before it is given final approval -but once it has passed a third reading in the Commons, a bill is pretty much certain to become law.

The Lords are expected to put up a fight over the so-called Dubs amendment, which seeks to guarantee the right of unaccompanied child refugees to be reunited with family living in the UK after Brexit. Labour's Lord Dubs, who came to the UK as a child fleeing the Nazis, has led calls for the provision to be included in the legislation.

Former prime minister Theresa May agreed to include it in her Brexit bill, but it has been stripped from Boris Johnson's version. A commitment to workers’ rights was also dropped from the new legislation.

On Wednesday, MPs voted down a Labour amendment in the Commons that sought to reintroduce the legal protection for child refugees.

Dubs has already said that the government will be challenged on the issue when the bill goes to the Lords next week.

In a process known as “ping-pong”, the bill then goes back to the Commons for MPs to consider the amendments put forward by the Lords. At present, the government has only tabled an hour to consider these amendments, but it could modify the timetable to allow more time.

The bill is expected to receive royal assent -meaning it officially becomes law -in the week beginning Jan. 20.

The European Parliament will then need to give its approval, and the UK will cease to be a member state on Jan. 31 -the date of the fourth Brexit deadline agreed between Britain and the EU.

There will be no immediate changes in the UK, as it will then enter a transition period until the end of the year, giving the government just 11 months to negotiate the all-important future relationship with the EU and trade deals with other countries.

May’s bill had included provisions for this transition period to be extended by Parliament, but under Johnson’s agreement, the date cannot be extended, meaning the UK will stop abiding by the EU’s rules and paying into EU budgets by the end of 2020 come what may.

December’s general election marked the end of more than a year of deadlock over the withdrawal agreement. In a series of fraught evening votes in the Commons, May repeatedly failed to get her bill through Parliament, with Eurosceptic members of her own party opposed to it on the ground that they felt it kept Britain too tightly aligned to the European Union.

May stepped down, allowing Johnson to take the helm -but with a tiny majority, he struggled to pass any legislation at all. However, his election gamble paid off, leaving him with a majority of 80 -more than enough to ensure that he can pass legislation with ease.

While Johnson’s deal has won the support of his party, it has been heavily criticised by the opposition. The new agreement introduces customs checks on goods crossing between Northern Ireland and the rest of the UK, effectively creating a border in the Irish Sea.

The agreement replaces May’s controversial Irish backstop, which would have kept the whole of the UK in a “single customs territory” with the EU if a free trade agreement had not been reached by the end of the transition period, but opposition parties say that a border in the Irish Sea threatens peace and stability in Northern Ireland.