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UK Prime Minister Theresa May

Thirteen of Theresa May’s former cabinet colleagues have written to her demanding she doesn’t strike a deal with Labour over Brexit.

The group includes former Defence Secretary Boris Johnson, Former Defence Secretary Gavin Williamson and Former Brexit Secretary Dominic Raab.

They warn that Mrs May could lose more Tory MPs than she gains Labour backers if she doesn’t leave the EU Customs Union as promised.

The letter adds: “More fundamentally, you would have lost the loyal middle of the Conservative Party.”

Sir Graham Brady, chairman of the backbench 1922 Committee, is also a signatory.

May meets with her cabinet on Tuesday to discuss Brexit. She is facing growing calls to announce her departure date.

“No leader can bind his or her successor so the deal would likely be at best temporary, at worst illusory,” the signatories said.

Michael Fallon added on Tuesday that the UK was better off staying in the EU if the deal included a permanent customs union.

Labour leader Jeremy Corbyn also faces growing calls to halt the talks.

Corbyn was further asked to clarify his position on Brexit at an all party Parliamentary meeting on Monday.

Olly Robbins, Mrs May’s chief Brexit negotiator, travels to Brussels on Tuesday meanwhile.

He will discuss potential changes to the future political declaration.

The EU is open to making changes, it says, but must know what the UK wants.