Tory MP Nick Boles is resigning from his local Conservative association after clashing with them over Brexit.
Mr Boles intends to remain as MP for Grantham and Stamford.
He has spoken out against the UK leaving the EU without a deal and has faced calls for his deselection over his Brexit stance.
In his letter, seen by the BBC, he said he was resigning with immediate effect and that a “division had opened up” between him and the local association.
Mr Boles said he wanted to continue to “take the Conservative whip” at Westminster if it is offered “on acceptable terms” – meaning he would vote with the party.
He wrote: “I regret that my relationship with you should end in this way. But a politician without principles is worthless.
“I am in no doubt about my duty, which is to be true to my convictions and to dedicate the rest of my time in Parliament to the best interests of the people I was elected to serve.”
Councillor Martin Hill, vice president of the Grantham and Stamford Conservative Association, told members: “As you are all aware, Nick has been at odds with the local party and the prime minister for some time, so this announcement does not come as a complete surprise, but the timing does leave at lot to be desired.”
‘Everything in my power’
Mr Boles had previously told the BBC that his constituency party was looking to oust him as a candidate at the next election.
He voted in favour of extending Article 50 in the Commons this week, and in favour of Theresa May’s Brexit deal.
In his letter, he said: “While I have consistently argued that Brexit must be delivered, and have voted for the prime minister’s deal every time she has brought it to the House of Commons, I am certain that crashing out of the EU without a deal would do great harm to the British people and have done everything in my power to prevent it.”
Mr Boles said he was “proud” of his role in the cross-party campaign to force Mrs May to request an extension to Article 50 beyond 29 March, the day the UK is due to leave the EU, and block a no-deal Brexit.
“In securing substantial Commons majorities in favour of both propositions last week, I believe we have done the country a great service,” he added.