Conservative MP Gareth Johnson has quit the government in order to oppose Theresa May's EU Withdrawal Agreement.
Mr Johnson announced he was resigning his post as an assistant whip, which he had held only since November 2018.
The Dartford MP is the latest in a string of Tories to walk out of the government in protest at the British Prime Minister's deal, and his resignation comes at a crucial moment ahead of Tuesday's "meaningful vote".
He announced his decision shortly after Mrs May made an eleventh-hour plea for MPs to back her deal, following the publication of a letter from EU chiefs offering assurances about the backstop.
In a letter to Mrs May, Mr Johnson said he had struggled to reconcile his duty as a whip to help push the deal through the Commons with his personal objections to the agreement.
He warned that the proposed backstop arrangements designed to avoid a hard border in Northern Ireland failed to provide the UK with a "clear, unilateral path out of the EU" and would ensure Britain was "fettered in our ability to negotiate trade deals".
Mr Johnson, who backed Leave in the 2016 referendum, wrote: "This agreement prevents us taking back control and instead could leave us perpetually constrained by the European Union."
He said it was now clear that "no significant change" would be made to the agreement, and he had therefore decided "to place my loyalty to my country above my loyalty to the government".
"We need to rediscover our confidence and belief in our country's ability to stand tall in the world without the European Union overseeing and managing our future," wrote Mr Johnson.
"This is possibly the hardest decision I have ever made, but I believe it is the right one for both my country and my constituents."
.@theresa_may warns of a no-deal Brexit or no Brexit at all if MPs don't back her deal pic.twitter.com/WUxKi47rqJ
— RTÉ News (@rtenews) January 14, 2019
Mr Johnson, 49, has been MP for Dartford since 2010 and served as parliamentary private secretary to former Brexit secretaries David Davis and Dominic Raab before being appointed an assistant whip in November.