Dominic Raab has resigned as Brexit secretary, claiming Theresa May's withdrawal agreement posed a "very real threat" to the integrity of the UK.
In his resignation letter, the Conservative MP for Esher and Walton wrote that his respect for the Prime Minister remained "undimmed".
However, he claimed he could not support an "infinite backstop agreement", adding the regulatory regime proposed for Northern Ireland presented "a very real threat to the integrity of the UK".
The politician posted a picture of his resignation letter on Twitter, writing: "Today, I have resigned as Brexit Secretary. I cannot in good conscience support the terms proposed for our deal with the EU.
"Here is my letter to the PM explaining my reasons, and my enduring respect for her."
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Mr Raab held the role of Brexit secretary for 138 days.
His resignation follows that of Northern Ireland Minister, Shailesh Vara, who quit his role this morning over Theresa May's "halfway house" Brexit agreement.
In a statement outside 10 Downing Street following last night's cabinet meeting, Ms May insisted the deal was "the best that could be negotiated".
"The choices before us were difficult, particularly in relation to the Northern Ireland backstop, but the collective decision of Cabinet was that the government should agree the draft withdrawal agreement and the outline political declaration," she said.