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."
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.
Here is the text of Gareth Johnson's letter of resignation:
14 January 2019
Dear Prime Minister,
It is with regret that I hereby tender my resignation to you as a Minister in the Government Whips' Office.
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 objections to the agreement. I have concluded that I cannot, in all conscience, support the Government's position when it is clear this deal would be detrimental to our nation's interests.
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.
Along with nearly 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 perpetually constrained by the European Union.
Like you, I am not only a Conservative but I am also a committed Unionist and I cannot accept the additional regulatory compliance required of Northern Ireland that would set it apart from the rest of the United Kingdom. I have prided myself on being a loyal Member of Parliament and I was very grateful to be given the opportunity by you last year to serve in the Government Whips' Office. I am also proud of the many achievements of this Government but I believe it would be disrespectful to the referendum result if this agreement were to be implemented. I have therefore decided the time has come to place my loyalty to my country above my loyalty to the Government.
I was hopeful that changes could be made to improve the deal with the European Union but it is now clear to me that no significant change will be made to this agreement before the Meaningful Vote takes place. I hope, however, that even at this late stage, efforts will continue to change the terms of this agreement and the impact it would have on our country. 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.
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.