A Labour Party activist yesterday admitted that he sparked the investigation into Ken Livingstone which led to the MP being condemned by the Commons' disciplinary committee.
The revelation sparked new claims that the independent candidate for London Mayor had been the victim of a dirty tricks campaign.
Rob Smeath, a London-based solicitor who worked on the London mayoral campaign of broadcaster Trevor Phillips,said he acted alone when he asked a friend to make the complaint about the Brent East MP's earnings from public speaking and media work.
But it was still unclear how Mr Smeath knew that Shaun Woodward, a Tory London spokesman who defected to Labour the day before the letter was written, had never lodged a complaint he promised three weeks earlier.
Mr Smeath told the London Evening Standard he had asked a friend to make the complaint for him because he did not want the issue to be linked with controversy over the mayoral contest.
He chose John Jones, an associate researcher at Loughborough University, because Mr Jones had no interest in politics. But Mr Jones became nervous after a critical report on Mr Livingstone was published and denied all knowledge of the letter.
Mr Smeath, 31, works for Simmons and Simmons in the City of London. He was treasurer for Mr Phillips' campaign before he joined forces with Frank Dobson and also acted as agent for Mr Dobson's campaign manager, Jeremy Fraser, in the 1997 election.
"I wanted to get the issue of MPs not declaring earnings discussed. I was so incensed. I had been involved in the Trevor Phillips campaign and I wanted to be distanced from any idea that Trevor was making me do this," he said.
He said he simply wanted to see the issue aired while getting on with his own life and allowing both Mr Dobson and Mr Phillips to get on with theirs. He had "a lot of time" for Mr Livingstone, he added.
Mr Smeath said Mr Jones had agreed to sign the letter of complaint to Elizabeth Filkin, Parliamentary Commissioner for Standards, "albeit reluctantly".
Last week the Standards and Privileges Committee endorsed a report from Ms Filkin on the issue and recommended that Mr Livingstone should make an apology to the House of Commons. The MP had earned £220,000 apart from his salary in an 18-month period and should have declared £158,000 of it, the committee said.
Mr Livingstone said he was not surprised by yesterday's revelation. "We knew that it was organised from someone of the Millbank Tendency," he said.
"We were just surprised that it was someone so close to Trevor and Frank. There will be lots more dirty tricks but I don't think the public is drawn in."
The Labour Party said Mr Smeath would not be making any further comment last night.
Yesterday Ms Filkin said she did not expect to take any further action on the issue as the letter of complaint had been signed by Mr Jones and had proved to be correct.
"It is apparent that from time to time MPs and members of the public can use the complaints system to have a go at people on the other side," she said.
"But if there is nothing in the complaint nothing will be upheld. I am only interested in fact - I am not interested in motive."