Gareth Johnson, who resigned his position in the Whips’ Office a short while ago, said the decision was hardest he had “ever made”.
The MP for Dartford – who backed Leave in the referendum – said he had been “hopeful that changes could be made to improve the deal with the European Union but it is now clear to me that no significant changes will be made”.
“Over the last few weeks, I have tried to reconcile my duties as a Whip to assist the Government to implement the European Withdrawal Agreement, with my own personal objection to the agreement,” Gareth Johnson wrote in a resignation letter to May.
“The ‘back stop’, contained in the agreement, gives our country no clear, unilateral path out of the European Union and ensures we will be fettered in our ability to negotiate trade deals with other nations in the future,” he added.
“Along with near, two thirds of my constituents and a majority of the country, I supported ‘leave’ in the referendum as I wanted the UK to take back the sovereignty we had lost during our membership of the European Union.
“Unfortunately, this agreement prevents us taking back control and instead could leave us perpetual, constrained by the European Union.”
The prime minister rewrote part of her speech about Brexit following criticism it was factually inaccurate.
Theresa May had planned to say that both sides had accepted the result of the Welsh assembly referendum in 1997.
But she had voted against the creation of the institution following the devolution referendum.
Labour and Plaid Cymru politicians accused her of hypocrisy – and the line, which had been given to journalists ahead of the speech, was dropped.
Instead, she said the result was accepted by parliament.