The developments came as Mrs May prepared for a crucial meeting with European Commission president Jean-Claude Juncker in Brussels.
"I have great respect for Theresa May, for her courage and her assertiveness".
"The EU 27 will not reopen the withdrawal agreement, we can not accept a time limit to the backstop or a unilateral exit clause", said Margaritis Schinas, spokesman for the EU's executive European Commission.
Mrs May is expected to request legally-binding assurances that the backstop will not extend indefinitely.
The three defectors from May's party said they were joining a new independent group in parliament set up this week by seven former lawmakers of the main opposition Labour Party, which is as divided over Brexit as the Conservatives.
May has until 27 February to secure European Union concessions on the backstop or face another series of Brexit votes in the House of Commons, where lawmakers want changes to the withdrawal deal.
Ahead of May's visit, Brexit Secretary Stephen Barclay and Attorney General Geoffrey Cox travelled to Brussels to make another attempt to renegotiate the controversial Irish backstop.
But Lord Callanan said: "The Prime Minister has been very clear that we intend to leave on March 29, that's what Article 50 says and that's what our domestic legislation says".
She has promised parliament to rework the treaty to try to put a time limit on the protocol or give Britain some other way of getting out of an arrangement which her critics say would leave the country "trapped" by the EU.
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Jamie McMurray, the 2010 victor , led six laps and was in the mix until he was collected in one of the late accidents. Until Sunday, Denny Hamlin hadn't won a NASCAR Cup Series race since September of 2017.
This provides for Britain to remain in the EU customs union until a way is found - such as a future free trade deal - to ensure that Ireland's border with Northern Ireland remains open.
A joint statement issued following the meeting said: "The two leaders agreed that talks had been constructive and they urged their respective teams to continue to explore the options in a positive spirit".
EU Commission President Jean-Claude Juncker told the German daily Stuttgarter Zeitung on Tuesday that such a scenario was "difficult to imagine", saying it would be a "belated joke of history".
Theresa May will head to Brussels on Wednesday armed with fresh legal proposals to change the contentious Irish backstop in a bid salvage her Brexit deal.
If the UK Parliament does not agree on the deal before March 29, Britain risks a chaotic departure that could be costly to businesses and ordinary people on both sides of the Channel.
Corbyn said he would use the meetings with European Union chief negotiator Michel Barnier and others to stress that a no-deal Brexit does not command a majority in parliament.
"Any decision to ask for more time lies with the UK".
But delaying Brexit would require the EU's approval - and if it is extended too far, it would force Britain to take part in the May 23-26 EU-wide election for the European Parliament.
EU Commission President Juncker told reporters earlier on Wednesday that he wasn't expecting a breakthrough in the talks.
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