Labour is to demand that Prime Minister Theresa May withdraws her Brexit legislation amendment which would set the day of EU departure in law.
Shadow Brexit secretary Keir Starmer will brand it a “gimmick” and warn that Labour will vote against it if it is not withdrawn by the government.
It comes as MPs begin debating the crucial piece of legislation that will pave the way for Brexit.
The European Union (Withdrawal) Bill will end the supremacy of EU law.
A government amendment that would enshrine the Brexit date and time – 11pm on March 29 2019 – in law, announced last Friday, by Prime Minister Theresa May, will not be debated until the final day of the committee stage.
Keir Starmer said Labour accepted that Britain would leave in March 2019 but setting a date in law was a “desperate gimmick” that was “about party management not the national interest”.
“The government’s amendments to their own Bill would stand in the way of an orderly transition and increase the chance of Britain crashing out of Europe without an agreement.
“Theresa May should stop pandering to the ‘no deal’ enthusiasts in her own party and withdraw these amendments. If not, Labour will vote against them to support our own amendments and guarantee a transition that protects jobs and living standards.”
BBC Political Editor Laura Kuenssberg said many Conservatives were also unhappy about the proposal and it is one of the areas where the government potentially faces defeat in the coming weeks.
What is happening on Tuesday:
Labour rebel Frank Field’s proposal to include a date of exit from the EU – March 30 2019 – was the first to be debated by MPs.
Defending his position, the Birkenhead MP said he had never taken on a job without a start date or bought a house without knowing when he would take possession.
He also called for the UK stop paying money into the EU until they “start seriously negotiating with us”.
Mr Field’s amendments will be considered alongside Plaid Cymru’s desire for the devolved legislatures in Wales, Scotland and Northern Ireland to be required to grant consent to allow the prime minister to repeal the European Communities Act 1972.
Ministers say the main aim of the EU (Withdrawal) Bill is to copy across EU rules into domestic UK law to ensure a smooth transition on the day after Brexit.
But critics say it is a power grab by the government which will allow ministers to change laws and regulations without going through Parliament first.
Most MPs say they accept that Britain is leaving the EU but some are expected to use the debates to fight against what they call a “hard Brexit” that would see the UK leaving the single market and customs union – or leaving without a trade deal.
MPs have tabled more than 470 amendments – running to 186 pages – for changes they want to see before the Brexit bill is passed into law.
A further seven days of debates have been scheduled in the run-up to Christmas, with the second day, on Wednesday, expected to include Labour’s calls for guarantees on workers’ rights and the environment.
The government is not thought to be facing the serious prospect of defeat until next month, with a small group of about 10 Conservative rebels reportedly plotting with Labour and other opposition parties to back critical amendments.
David Davis said on Monday that MPs would be able to debate and vote on any agreement negotiated with the EU by the government – because, he said, the government had decided that the Brexit deal would have to become law via an Act of Parliament.
But he said the UK would still leave the EU on 29 March 2019, whether MPs backed or rejected the deal – making MPs’ vote a take-it-or-leave-it one on the Brexit deal, rather than one which could either halt Brexit or have the deal renegotiated.
The promise of a vote on any final deal was welcomed by Labour and some Tory backbenchers on Monday, with Keir Starmer calling it a “significant climbdown from a weak government on the verge of defeat”.
But Conservative MP Anna Soubry said the government’s offer was “completely meaningless” because Mr Davis “couldn’t guarantee that we’d have the vote before we left”.
And if the government failed to reach an agreement with the EU on its post-Brexit relationship Parliament will have no say at all, she added, which was not “right” for such an important decision.
What stage is the bill at?
The EU Withdrawal Bill is entering its Committee stage – meaning MPs will scrutinise it line-by-line in the House of Commons.
The debate is set to last for two days this week, although no crucial votes are expected.
There will be six more days of debate at a later date. But if the bill passes this stage, it still has a long way to go.
It will return to the House of Commons with any additional amendments for its report stage and then will have a third reading in front of MPs.
The bill will then have to go to the House of Lords for scrutiny before it can gain Royal Assent and become law.