* Britain due to leave EU on March 29
* So far, no divorce deal in place
* May has told EU with changes she can get deal passed
* May facing rebellion by pro-Brexit Conservatives (Adds Barclay comments, amendments, ERG)
By Elizabeth Piper, Kylie MacLellan and William James
LONDON, Feb 14 (Reuters) - British Prime Minister Theresa May could face a defeat in parliament on Thursday over her plan to renegotiate the Brexit deal, undermining her pledge to the European Union that, with changes, she can get the agreement approved.
Thursday's symbolic vote was seen by May's team as little more than a rubber stamp of her plan to secure changes to the divorce deal with the EU, giving her more time to satisfy lawmakers' concerns over one part of it - the Irish backstop.
But hardline Brexit supporters in her Conservative Party are angry over what they say is her acceptance of ruling out a no-deal departure, something May's Brexit minister Stephen Barclay again denied in parliament, saying by law Britain will leave the EU on March 29 with or without an agreement.
The latest twist in the two-year negotiation to leave the EU underlines the deep divisions in parliament over how, or even whether, Britain should leave the bloc in the country's biggest political and trade policy shift in more than 40 years.
A rebellion, even in a symbolic vote, would be a blow to May, who has insisted to EU leaders that if they offer her more concessions to the deal agreed in November, she can command a majority in parliament and get the agreement passed.
"Colleagues should be in no doubt that the EU will be watching our votes tonight carefully for any sign that our resolve is weakening, we should not give them that excuse not to engage," Barclay told parliament as he opened the debate. Lawmakers are expected to vote from 1700 GMT.
A government source put it more bluntly. "Without support from MPs, it will be harder for the government to get the changes to the backstop we know they want," the source said.
Steve Baker, a member of the pro-Brexit European Research Group of Conservative lawmakers, said no lawmaker in the governing party should be associated with anything which seems to take a "no-deal Brexit" off the table.
One Conservative lawmaker said the ERG was still discussing which strategy to pursue on Thursday after speaking with the government's chief whip, May's enforcer in parliament. Two reporters quoted sources as saying the group would abstain, most likely handing the prime minister a defeat.
May is trying to secure changes to the backstop arrangement to prevent a return of border controls between Northern Ireland and EU-member Ireland to ease concerns that Britain will be kept too closely in the EU's orbit indefinitely or that the British province will be split away.
On Wednesday, European Council President Donald Tusk said the bloc was waiting for Britain to present solid proposals to break the impasse after meetings in Brussels and telephone calls between May and EU leaders.
Some Conservative and many opposition lawmakers accuse May of "running down the clock", edging Britain closer to the exit date to try to force parliament to choose between backing her deal or leaving without an agreement.
Many businesses say that outcome would be catastrophic for the world's fifth largest economy by causing delays at ports, fracturing international supply chains and hindering investment.
More than 40 former British ambassadors called on the government to extend Britain's stay in the EU or allow for a second referendum, The Times newspaper reported. May has repeatedly said she does not back a second vote.
To try to prevent a no deal, several lawmakers have put down alternative proposals.
Parliament's speaker selected three which can be put to a vote - one which calls for parliament to be given a vote on a revised Brexit deal or a debate on the next steps by Feb. 27, another which seeks to delay Brexit for at least three months and third which calls on the government to publish an impact assessment of no deal.
It is not clear whether any of the three will win enough support to pass, with lawmakers who are seeking to force the government to delay Brexit saying they will wait until the next round of votes May has promised on Feb. 27 to make their move.
(Additional reporting by Andrew MacAskill and James Davey; Editing by Janet Lawrence)
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