Brexit: MPs voting on Johnson's Brexit bill
MPs are voting on whether to back the PM's plan for the UK to leave the EU on 31 January.
The EU (Withdrawal Agreement) Bill would also ban the government from extending the transition period - where the UK is out of the EU but follows many of its rules - past 2020.
Boris Johnson said it would allow the UK to "move forward".
But Jeremy Corbyn said Labour would oppose the bill, and there was "a better and fairer way" to leave the EU.
Mr Johnson has insisted a trade deal with the EU can be in place by the end of the transition period, but critics say this timetable is unrealistic.
The result of the Commons vote is expected at about 14:30 GMT.
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The withdrawal bill, which would implement the Brexit agreement the prime minister reached with the EU in October, was introduced in Thursday's Queen's Speech, setting out the government's priorities for the next year.
Beginning the debate in the Commons, the prime minister said his bill "learns the emphatic lesson of the last Parliament" and "rejects any further delay".
"It ensures we depart on 31 January. At that point Brexit will be done. It will be over," he told MPs.
"The sorry story of the last three years will be at an end and we can move forward."
Mr Johnson said it also "paves the way" for a "ambitious free trade deal" with the EU.
The bill's second reading is the first chance MPs have had to debate its main principles in the House of Commons.
With the Conservatives having won an 80-seat majority at last week's general election, the bill is expected to pass easily, before it moves on to further scrutiny by MPs and the House of Lords.
MPs have been given a further three days - 7, 8 and 9 January - to continue their debate.
The government says it will get the bill into law in time for the 31 January Brexit deadline.
There are changes to the previous bill, which was backed by the Commons in October, but withdrawn by the government after MPs rejected a three-day deadline for getting it through Parliament.
The changes include:
- Legally prohibiting the government from extending the transition period - during which a trade deal between the UK and EU will be discussed - beyond 31 December 2020
- Allowing more UK courts to reconsider European Court of Justice rulings that have been retained in UK law after Brexit
- Requiring ministers to report annually to Parliament on disputes with the EU under the prime minister's withdrawal agreement
- Repealing spent legislation that "now serves no purpose"
The bill also loses a previous clause on strengthening workers' rights.
The government now says it will deal with this issue in a separate piece of legislation, but the TUC has warned that the change will help "drive down" working conditions.
What a difference a year makes.
It was back in January that Theresa May embarked on a series of Commons defeats as she tried and failed to begin the process of getting her Brexit plans approved.
It was only in October that Boris Johnson paused his own efforts when MPs rejected the proposed timetable for getting the Withdrawal Agreement through parliament.
But now, following the general election and with an 80-strong Conservative majority, things look very different.
And Boris Johnson knows it, claiming that it's time for "certainty" after years of "delay and rancour".
But the bill will come in for criticism. Gone are clauses about workers' rights - Downing Street says that will be dealt with in separate legislation.
And added: a provision ruling out any extension to the transition period beyond December 2020.
The process of ratifying the Withdrawal Agreement Bill will continue in the New Year but Friday's vote is, in part, designed to signal that the UK is now motoring towards that January 31 departure date.
Labour leader Mr Corbyn said the government's "mishandling of Brexit" had "paralysed the political system," divided communities and was a "national embarrassment".
He said MPs "have to respect the decision" of the EU referendum in 2016 "and move on".
"However, that doesn't mean that we as a party should abandon our basic principles," he said.
"Labour will not support this bill as we remain certain there is a better and fairer way for this country to leave the EU."
He said there had to be something better than this "terrible" Brexit deal that would not "sell out public services" or "sacrifice hundreds of thousands of jobs in the process".
However, Labour MP for South Shields, Emma Lewell-Buck, said she would vote for the bill, adding: "The party opposite have a mandate they did not have before.
"It is with the heaviest of hearts that I cannot vote with my party today but I will always put my constituents first."
The SNP's Westminster leader, Ian Blackford, said: "Scotland still totally and utterly rejects Brexit, yet the prime minister is blindly hurtling towards the cliff edge with these Brexit plans that will leave us poorer, leave us worse off."
On the change in the bill that would legally prohibit the government from extending the transition period beyond 31 December 2020, Mr Blackford said: "By placing that deadline, that risk of a no-deal Brexit, that we all fear is very much, is on the table again."
And the Democratic Unionist Party's Sir Jeffrey Donaldson said there was a "major contradiction" in the prime minister's deal "that causes us great concern".
He said, while it mentioned "unfettered access" for Northern Ireland when it comes to trade in the UK, it also had customs arrangements "that inhibit our ability to have that unfettered access".
Meanwhile, Labour 's Lord Dubs said it was "appalling and deeply distressing" that his amendment to the previous Brexit bill, which proposed that the UK would continue to enable unaccompanied child refugees to be reunited with their families, had been removed.
But Mr Johnson said: "We remain proud of our work in receiving unaccompanied children. We'll continue to support fully the purpose and spirit of the Dubs amendment but this is not the place, in this bill, to do so."
In the 2016 referendum, the UK voted by 52% to 48% to leave the EU. But the subsequent difficulties in getting Brexit through Parliament have caused gridlock at Westminster.
An earlier withdrawal agreement - reached between previous Prime Minister Theresa May and the EU - was rejected three times by MPs.