The Commons Standards and Privileges Committee, which examines disciplinary complaints, last night agreed a report into an allegation that Mr Mitchell exerted improper influence when he served as a Government whip on the old Commons Committee on Members' Interests.
It had been claimed that Mr Mitchell - like Mr Willetts - had attempted to subvert an inquiry into an allegation that his fellow Tory MP Neil Hamilton had accepted cash for asking parliamentary questions. Last night's report is thought to have concluded that Mr Mitchell behaved "inappropriately".
But the strongest criticism was said to have been diverted, and directed at Richard Ryder, the former Government Chief Whip, for allowing Mr Mitchell, one of his team, to be placed on the Members' Interests Committee in the first place.
Mr Ryder, who is standing down from Parliament at the election, was said last night to be unconcerned by the criticism. One friend said, quite rightly, that it was not Mr Ryder who had placed Mr Mitchell on the committee - but the Commons itself.
Mr Mitchell is thought to have "saved" himself from severe censure by taking a more apologetic tone than Mr Willetts when he was questioned - on oath - in a public hearing on 20 January.
He told the Standards and Privileges Committee that "with hindsight" it would be better if government whips did not sit on committees which have a quasi-judicial role. Mr Mitchell acknowledged that there had been a potential conflict of interest between his job as a whip and his membership of the Members' Interests Committee.
Appointed to the committee in June 1994, Mr Mitchell said in evidence that he had not appreciated that it sat in a quasi-judicial capacity; making judgement on colleagues' conduct.
The report, which is expected to be published tomorrow, is thought to endorse that view.
But it was also said to accept that there was no evidence of a whips' office conspiracy to rig the Hamilton investigation.