Senior Tory Nick Boles has resigned from his local Conservative Party amid a heated row over Brexit and a drive to deselect him as the seat’s candidate for the next election.

The former minister told members of Grantham and Stamford Conservative Association he was resigning “with immediate effect” but will continue to take the Conservative whip at Westminster if it is offered “on acceptable terms”.

His departure serves to underline the disintegrating unity of his party following a week that saw 188 Tory MPs, including eight cabinet ministers, refuse to back Theresa May’s strategy to delay Britain’s departure from the EU.

Next week the prime minister will again try to push her Brexit deal through the House of Commons, after it was defeated on Tuesday by 149 votes and by 230 votes in January.

Mr Boles was a close ally of former prime minister David Cameron, supported Remain in the 2016 referendum, and has been an advocate since of remaining in a customs unions and single market with the EU.

While he has backed Ms May’s deal in votes, he has enraged his local association by also taking part in cross-party moves to strip her executive of power to set parliament’s agenda, with the goal of giving MPs a chance to choose the next Brexit steps should her deal fail. Many Conservative Eurosceptics see this as an attempt to soften or even stop Brexit altogether.

The MP’s letter of resignation, seen by the BBC, suggests he will still be prepared to vote along party lines in some instances and could still back Ms May’s deal in a critical vote on Tuesday.

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 a lot to be desired.

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“I understand that it is Mr Boles’s intention to carry on as the MP for Grantham and Stamford until the next general election despite being elected to represent us in parliament.”

After losing votes on previous occasions in the House of Commons, the prime minister was forced to let MPs vote on extending Article 50 last Thursday.

The motion put in front of the house said she would go to the European Council at the end of next week and ask for a short delay to the end of June if MPs backed her deal and a longer one if they did not.

But with Conservative ranks full of MPs who baulk at the idea of delaying the UK’s departure at all, and the prospect of having to sack dozens of minsters, she allowed a free vote.

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A further 27 junior ministers and 11 whips also voted against the approach Ms May is taking, including the outspoken Conservative deputy chairman James Cleverly and other Brexit ministers Kwasi Kwarteng and Chris Heaton-Harris. Three other whips, including chief Julian Smith, declined to vote.