MP Patrick Mercer has resigned the Tory whip to "save my party embarrassment" over a BBC Panorama programme alleging he had broken lobbying rules.
The Newark MP said he will not be standing at the next general election but there will not be a by-election.
Mr Mercer, who has referred himself to the parliamentary standards commissioner, said he was "taking legal advice" about the allegations.
The Panorama programme will be aired "as soon as possible", the BBC said.
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."
MPs who resign the whip can continue to sit in the Commons as independents but are no longer members of the parliamentary party.
'Done right thing'
A Conservative 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."
Reacting to the news, Conservative MP Richard Ottaway told the BBC: "I'm very sorry to hear this.
"Patrick Mercer has been a good servant of the party for decades. The fact that he's going, I'm really disappointed over. I have yet to hear the reasons, but it's a sad day."
In a statement, the BBC said: "BBC 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."
Race row
Mr Mercer, a former army officer and shadow homeland security minister, was sacked from the Tory front bench by David Cameron in 2007 after making allegedly racist comments.
It followed an interview in The Times, in which he said: "I came across a lot of ethnic minority soldiers who were idle and useless, but who used racism as cover for their misdemeanours."
During the furore that followed he denied being racist but accepted his comments might have "hurt" soldiers who served with him and "embarrassed" his party.
Later that year he was made an adviser on security matters to Labour minister Lord West.
The coalition government is committed to setting up a statutory register of lobbyists - companies that influence government policy, often by paying current and former MPs for advice and guidance.
Before the 2010 election, Mr Cameron predicted that it would be the next big scandal to hit British politics, but the policy, which is in the coalition agreement, has yet to become part of the government's legislative programme.
BBC News
Tip of the iceberg, 99% of these cunts we pay are in the pockets of big business lobbying and looking after the interests of companies not the people who put them there.
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