FILE PHOTO - Britain's Labour Shadow Foreign Secretary Emily Thornberry arrives at BBC Televsion studios to appear on the Marr Show in central London, Britain, June 17, 2018. REUTERS/Peter Nicholls

LONDON (Reuters) - Britain’s opposition Labour Party is set to vote against any Brexit deal reached by Prime Minister Theresa May and the lack of a viable exit from the European Union will force May from office before Christmas, a senior Labour lawmaker told the Financial Times.

The United Kingdom is due to leave the European Union on March 29 and yet little is clear. There is, so far, no full exit agreement and some rebels in May’s Conservative Party have threatened to vote down a deal if she clinches one.

If rebels in her party do vote against a deal, the fate of May’s government and the whole exit process would depend on Labour because she would not command the 320 votes needed to carry parliament.

Emily Thornberry, Labour’s foreign secretary in waiting, told the FT that a workable deal was “just not going to happen” under May.

Labour has set six tests for supporting any Brexit deal but Thornberry said she saw little chance of any May deal satisfying them.

“I can’t see them coming back with a deal that is going to meet our six tests and I can’t see them coming back with a deal that will unite the Tory party, for Heaven’s sake ,” Thornberry told the FT.

Thornberry said there would need to be a national election within months given the likelihood that the prime minister would be defeated on the crucial vote on any Brexit deal.

“They are not capable of governing . . . We’re either going to have a general election in the autumn or we’re going to have it in the spring,” she said.

Thornberry said she wanted a national election rather than another referendum.