Tory MP Nick Boles is resigning from his local Conservative association after clashing with them over Brexit.
Mr Boles, who wants to remain as MP for Grantham and Stamford, has spoken out about leaving the EU with no deal.
Local activists had wanted to deselect him as their candidate in the next general election because of his 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.
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.”
Mr Boles said he wanted to continue to “take the Conservative whip” at Westminster if it is offered “on acceptable terms” – meaning he would still vote with the party.
Councillor Martin Hill, vice president of the Grantham and Stamford Conservative Association, told members they had been “betrayed by their parliamentary representative” and called on him to take the “honourable course” and quit as an MP.
He wrote: “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 a lot to be desired.”
He said the process of selecting a new candidate would start at the group’s AGM later this month.
Chief Whip Julian Smith said Mr Boles was a “valued member of the Conservative parliamentary party which I hope will continue to benefit from his ideas and drive”.
His announcement comes after a busy week in Westminster, when MPs voted to seek a delay to the UK’s departure from the EU, due to take place on 29 March.
The third “meaningful vote” on Prime Minister Theresa May’s deal is expected to take place next week. If it is agreed, she has promised to seek a short extension to the departure date. But if it doesn’t gain support, she has warned a longer extension may be needed – and the UK might have to take part in European elections.
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‘Everything in my power’
Mr Boles had voted in favour of extending Article 50 in the Commons this week, and in favour of Mrs 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 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.
Mr Boles is keen on a closer Norway-style relationship with Europe after leaving the EU.
But Mr Hill told the BBC: “Talk to the man and woman on the street and they’re also quite angry that their MP seems to be going back on what he promised to do at the general election. He signed up to the manifesto about coming out of the single market and the customs union.”
Meanwhile, pro-Brexit marchers, led by former UKIP leader Nigel Farage, have begun a two-week journey from Sunderland to London.
About 100 people had assembled to start the march. They were joined by counter-protesters, including those from anti-Brexit campaign Led by Donkeys.
Mr Farage aims to walk 100 of the 270-mile March to Leave, which is due to arrive in the capital on 29 March.
Separately, shadow chancellor John McDonnell has said politicians will move “heaven and earth” to prevent the UK from leaving the EU without a deal.
Speaking before an event in Gravesend, Kent, Mr McDonnell said “quite a number” of MPs would be prepared to support a compromise deal, with the guarantee the deal goes back to the people for a final say on Brexit.
Any final say should not be on the deal Mrs May has agreed, “because it’s not credible”, he said.