Backbencher Bob Spink has left the Tory group of MPs at Westminster, in a row over apparent efforts by his local party to deselect him.
Mr Spink is MP for Castle Point
Mr Spink said he had resigned the whip because the party had not taken action to sort out the association.
But the party said it withdrew the whip when he threatened to resign, in order to maintain "party discipline".
The Castle Point MP said he would fight the next election as an "independent Conservative".
The BBC understands he was facing a re-selection meeting next Tuesday.
Mr Spink raised a point of order during a debate on the Budget, saying he had "resigned the Conservative Party whip because the Conservative Party has failed to deal with serious criminal and other irregularities in my constituency".
But the party says it withdrew the whip - excluding him from the Tory group at Westminster - when he emailed the chief whip threatening to resign, accusing the party of "not doing enough to protect him during efforts by his association to deselect him".
In a letter the Conservative Chief Whip, Patrick McLoughlin, replied: "As a matter of good party discipline I cannot have MPs making threats to resign the whip at a time of their own choosing, if the demands of the party are not met.
"I must therefore treat your resignation as taking immediate effect".
Last month Mr Spink claimed members of his local Conservative Association had been working to deselect him.
He wrote in an article for his local paper: "It is for Castle Point residents to decide who will be their MP, not a small number of self-selected individuals with their private agendas."
In his letter, Mr McLoughlin said the party had initiated an investigation last year into the management of the local Conservative Association in response to Mr Spink's concerns.
"It did not find evidence to enable it to conclude that it would be justified in depriving the association of its rights under the constitution to consider your re-selection."
But Mr Spink told the BBC later: "If the party refuses to take action to sort out my local association, I don't see how I can be associated with it."
He said it was with "great sadness" he felt compelled to resign the whip and said he remained a "committed and principled Conservative" and would fight the next election as an independent Conservative.
Mr Spink is the third Conservative MP to lose the Tory whip in the last year, the others being Derek Conway and Andrew Pelling. Quentin Davies defected to Labour in June 2007.
Mr Spink has sometimes been a controversial figure. During the 2005 general election campaign he was accused of whipping up racial tension for running a newspaper advert on immigration headlined: "What bit of 'send them back' don't you understand Mr Blair?"