Newark MP Patrick Mercer has said he is quitting Parliament amid allegations that he broke rules on lobbying.
The backbencher said he was resigning the Tory whip immediately “to save my party embarrassment”, and would not stand at the next general election.
The move comes ahead of a BBC Panorama programme due to be broadcast next week.
In a statement, Mr Mercer said: “Panorama are planning to broadcast a programme alleging that I have broken Parliamentary rules.
“I am taking legal advice about these allegations – and I have referred myself to the Parliamentary Commissioner for Standards.
“In the meantime, to save my party embarrassment, I have resigned the Conservative Whip and have so informed Sir George Young. I have also decided not to stand at the next general election.”
A Conservative Party spokesman said: “The Prime Minister is aware. He thinks Patrick Mercer has done the right thing in referring himself to the Parliamentary Commissioner for Standards and resigning the whip.
“It’s important that the due processes take their course.”
A BBC spokesman said: “Panorama has been investigating lobbying and the conduct of MPs and members of the House of Lords.
“The programme is still being made and will be broadcast as soon as possible. The investigation has raised a number of issues related to those involved.
“Panorama has sought responses from a number of people including Mr Mercer.”
The Daily Telegraph said it had also been carrying out an investigation into Mr Mercer.
The MP and former army officer served as shadow homeland security minister until 2007, when he resigned after suggesting that racism was “part and parcel” of life in the forces.
Relations with David Cameron have been tense ever since, and in 2011 Mr Mercer was reportedly taped describing the Prime Minister as “despicable”.
Councillors in Mr Mercer’s constituency, where he has a 16,000 majority, said they were shocked at the news of his resignation.
Councillor Peter Duncan said: “I always thought of him as a very solid citizen. If this news is true then I am shocked. I thought he was a very strong individual with a strong point of view. He will be missed.”
Another Tory councillor Derek Evans said: “I’m very surprised at the news. I’ve a lot of trust and faith in Patrick. I have utter faith in him as an MP.”