But it has infuriated the hard-core Europhiles in the party, who want him to abstain or oppose when the pact goes before the Commons next week. Sir Keir is braced for frontbench resignations from junior shadow ministers who would rather quit than back the deal. The party was left bitterly divided over Brexit and many supporters in its traditional heartlands switched to the Conservatives at the last election in protest.
But many of Labour’s MPs are still fiercely opposed to Britain’s departure and do not want to put their names to the agreement.
Sir Keir said it was “in the national interest” to support the agreement despite concerns over the terms negotiated by the Government.
“At a moment of such national significance, it is just not credible for Labour to be on the sidelines,” he said.
Speaking after the deal was struck on Christmas Eve, he said that no deal would lead to “devastating” social, economic and political consequences, and said it was not right for Labour to abstain.
Sir Keir denied the suggestion the decision to vote for the deal to appease of large swathes of Brexit-supporting Labour voters at the last general election.
He said: “These are difficult and tough decisions.
“But in the end there is only one choice – a binary choice here.
“Either we support the deal or we support the alternative, which is no deal.
“We have always been against no deal and that is why we will vote for this deal.
“I think many people will see this as a tough but necessary decision on behalf of the Labour Party, the Labour movement and on behalf of our country.”
Parliament has been recalled for an emergency sitting on December 30 to vote on the free trade agreement and put it into UK law.
MPs will begin debating at 9.30am and can take part and vote remotely. Peers will start their scrutiny at noon on the same day.