By Elizabeth Piper, Andrew MacAskill and William James
LONDON (Reuters) - Britain's Brexit minister told parliament on Tuesday to back down in a showdown over Prime Minister Theresa May's plans for leaving the European Union, warning lawmakers they could undermine negotiations with Brussels.
Lawmakers began two days of debate on Tuesday on changes to May's Brexit blueprint, or EU withdrawal bill, after the upper house of parliament, the House of Lords, introduced 15 changes. The strained parliamentary session underlined deep divisions over Britain's EU exit.
The main point of contention between those who want to keep the closest possible ties with the EU and those who aim for a clean break is a demand to give parliament a "meaningful vote" on any agreement that May negotiates with Brussels.
Hours before the debate began, a justice minister resigned in protest at what he called its "wish to limit" the role of parliament in shaping Brexit. He said he would vote against the prime minister.
But the government moved quickly to bring onside one of the leaders of a group of Conservative lawmakers who are threatening to vote against May. The government's solicitor general, Robert Buckland, offered lawmaker Dominic Grieve a concession to discuss further improvements to the "meaningful vote" amendment.
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Brexit minister David Davis issued a warning to parliament over the possibility of rejecting the government's compromise on the "meaningful vote" and backing the House of Lords amendment.
"What it actually amounts to is an unconstitutional shift which risks undermining our negotiation with the European Union," he said. "The government cannot demonstrate the flexibility necessary for a successful deal if its hands are tied midway through that process."
Parliament is due to vote at around 1500 GMT in the first of two days of debate that will test May's authority.
In a highly charged atmosphere in parliament, lawmakers who oppose the government said they had received death threats and brandished a copy of the Daily Express newspaper, which ran a headline saying: "Ignore the will of the people at your peril".
Britons voted 52 percent to 48 percent in favor of leaving the EU in a June 2016 referendum.
Following days of frantic lobbying by Conservative officials, May renewed appeals for unity over the "meaningful vote", after the government appeared to have secured a compromise to stop a similar rebellion on Wednesday over Britain's trading ties with the EU.
Parliament must decide whether to support an amendment approved by the House of Lords that could mean sending May back into negotiations with the EU if lawmakers reject a Brexit deal.
Davis told lawmakers the government would never allow them to "reverse Brexit" and called on them to back its own amendment, which proposes a 28-day breathing space if parliament rejects a Brexit deal, during which the government would have to make a statement on its plans.
Grieve had offered his own competing amendment, which rebels in the Conservative Party had shown support for. His amendment could force ministers to hand over control of Brexit strategy to parliament if there is no deal by mid-February. The government earlier had said it would not support that amendment.
But around an hour before the vote was due, Buckland offered Grieve a compromise to discuss parts of his amendment that the government could adopt - a move aimed at winning the lawmakers and other rebels over.
Conservative Brexit campaigners have accused those in the party who indicated they would vote against the government of not respecting the referendum result.
The resignation by Phillip Lee, who has long been critical of the government's Brexit strategy, underlined the deep rifts in the party makes such votes anything but easy.
The "meaningful vote" will be the first major test after the House of Lords introduced changes to the bill, trying to reshape the government's approach to Brexit by encouraging lawmakers to press for the closest possible ties.
On Tuesday, parliament may vote on other amendments, including a challenge to the government's plan to put March 29, 2019, or 'Brexit Day', into law and an attempt to toughen a pledge to ensure a frictionless border between Northern Ireland and the Irish Republic, which will remain in the EU.
On Wednesday, parliament will consider a challenge to May's commitment to leave the EU's single market and customs union, which will transform Britain's future trading relationships for many years to come.
If May is defeated in the House of Commons it will be yet another blow to a prime minister whose authority has been challenged several times since last year's election. She now relies on the support of a small Northern Irish party.
"What would be the result to our government if we lost this vote today?" asked Conservative lawmaker Edward Leigh. "It would be a catastrophic blow."
(Additional reporting by Michael Holden; Editing by Janet Lawrence and David Stamp)