Newark MP Patrick Mercer has announced he is quitting parliament amid allegations he broke lobbying rules.
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 came after he was caught up in a joint sting by the BBC's Panorama programme and the Daily Telegraph.
The investigation is believed to have focused on Mr Mercer's alleged lobbying on behalf of Fiji – a country he tabled Commons questions on last month.
In a statement, the former shadow minister said: "Panorama is 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 chief whip Sir George Young.
"I have decided not to stand at the next General Election."
Mr Mercer's relations with David Cameron have long been fraught, and as news leaked out yesterday morning there was initially speculation he could be quitting in protest or defecting to the UK Independence Party (UKIP).
However, a Tory spokesman said the Prime Minister thought the MP had "done the right thing".
"It's important the due processes take their course," the spokesman said.
The former Army colonel served as shadow homeland security minister until 2007, stepping down after suggesting that racism was "part and parcel" of life in the forces.
Downing Street will hope to avoid a by-election in Newark, where Nigel Farage's UKIP could pose a threat despite the 16,000 majority secured in 2010.
However, pressure for the MP to leave the Commons immediately could grow when the Telegraph publishes details of the allegations today, while Panaroma is expected to air on Monday.
Conservative backbencher Zac Goldsmith said the case highlighted the need for voters to get powers to force by-elections – a change promised by the coalition but seemingly stalled.
"If it's bad enough for you to resign from your party, how can it be okay to continue representing constituents at all? Where's that Recall?!", he posted on Twitter.
Parliamentary records show Mr Mercer asked questions last month about Fiji's suspension from the Commonwealth, and UK investment in its public transport.
In March, he put down an Early Day Motion – a parliamentary device used to draw attention to issues – saying there was "no justification for Fiji's continued suspension from the Commonwealth".
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."