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Two vice-chairs of the Conservative Party are quitting their posts in protest at Theresa May's Chequers Brexit compromise plan.
Maria Caulfield and Ben Bradley warned the PM her plan would not capitalise on the opportunities of Brexit.
"This policy will be bad for our country and bad for the party," Ms Caulfield said.
Foreign Secretary Boris Johnson and Brexit Secretary David Davis have already quit over the proposals.
Mrs May says the proposed new relationship with the EU will "honour the result of the referendum" and allow the UK to "take back control of our borders, our law and our money".
It would see the UK agreeing a "common rulebook" with the EU for trading in goods, in an attempt to minimise friction for trade at borders.
But some Tories are unhappy, saying it will leave the UK tied closely to EU rules and prevent it from striking its own trade deals in years to come.
More details are expected on Thursday in a White Paper.
Ms Caulfield, who was vice chairwoman for women, supported Leave in the EU referendum. Mr Bradley - who backed Remain - had a brief trying to appeal to young voters. The Conservatives have nine vice chairmen in total, appointed in January's reshuffle.
In her resignation letter to Mrs May, Ms Caulfield said the proposed new trading relationship with the EU did not "fully embrace the opportunities that Brexit can provide".
Mr Bradley said he could not "with any sincerity defend this course" to voters in his Leave-backing constituency of Mansfield.
He criticised the decision to adopt a "backstop" position on customs to avoid a hard border in Northern Ireland, saying it was becoming "the barrier to the kind of wide-ranging free trade agreement with the EU that many in our party and the country would like to see".