Labour MPs face a “difficult decision” as Parliament debates whether to authorise the UK’s departure from the EU, its Brexit spokesman says.
Sir Keir Starmer said “as democrats” Labour should not block the start of the process but called for an end to “gloating” from Brexit campaigners.
He was speaking as MPs began a two-debate on legislation to authorise the triggering of formal negotiations.
The government wants to do this by the end of March.
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The European Union (Notification of Withdrawal) Bill would allow Prime Minister Theresa May to invoke Article 50 of the Lisbon Treaty, getting official talks between the UK and the EU started.
The Liberal Democrats and Scottish National Party are to vote against it, but Labour’s leadership is backing it, meaning the government is expected to win.
Brexit Secretary David Davis said the draft legislation was about “implementing a decision already made” by voters in the EU referendum.
He added that people would view any attempt to halt the Bill’s progress “dimly”.
With 99 MPs wanting to speak, discussions on the European Union Bill have been extended to midnight on Tuesday to accommodate more speakers, with a vote to take place on Wednesday evening.
Labour leader Jeremy Corbyn has ordered his MPs to vote with the government, but some are expected to defy him.
Sir Keir said two thirds of Labour MPs represented constituencies that voted to leave the EU, and one third where people voted to stay in.
“This is obviously a difficult decision,” he said.
“I wish the result had gone the other way. I campaigned passionately for that. But as democrats our party has to accept that result and it follows that the prime minister should not be blocked from starting the Article 50 negotiations.”
Former Conservative chancellor Ken Clarke confirmed he would vote against the bill, saying it was in the national interest for the UK to stay in the EU.
But his fellow Conservative Sir Bill Cash said the bill would restore the UK’s sovereignty and endorse the “massive peaceful revolution” of the EU referendum.
The size of the Labour rebellion will be closely scrutinised, with several of his MPs indicating they plan to defy Mr Corbyn.
Two shadow ministers have quit, saying they want to vote against it.
Although Labour’s leadership ultimately backs the draft bill the party will also attempt to amend the legislation, hoping to force the government into offering MPs a “meaningful vote” on the eventual Brexit deal reached after the negotiations.
If the vote goes the government’s way, the bill will return to the Commons next week for the committee stage when opposition parties will try to push through a series of amendments.
Speaking in Dublin on Monday, Mrs May said MPs would face a very clear choice when they came to vote on the bill.
“The people of the United Kingdom voted on 23 June last year. They voted in a referendum that was given to them overwhelmingly by Parliament,” she said.
The bill was published last week, after the Supreme Court decided Parliament must have a say.
Guy Verhofstadt, the EU parliament’s Brexit negotiator, told the BBC’s Newsnight programme Mrs May would not be allowed to “pick and choose” those benefits of EU membership she might want the UK to retain.
“We need a fair partnership,” he said. “You cannot create a status for countries outside the European Union where it is even more favourable than for the countries that are members. No taxpayer in Europe will accept such an outcome.”
But he said Europe needed to be “generous” not just to the UK as a country, but, in his opinion, to those UK citizens who wanted to remain in Europe.
“Maybe some advantages of the European Union could be kept for those people in the UK who want to have them in the future and that is a generous offer, I think.”