The Conservative Party board will meet today as pressure mounted on chairman Lord Feldman to quit over the handling of bullying and blackmail allegations.
The row over alleged behaviour by Mark Clarke shows little signs of abating despite the resignation of Grant Shapps, who was co-chairman with the peer until the general election.
The focus has now shifted to Lord Feldman after the departure of both men was demanded by the father of Elliott Johnson, a Conservative Party activist who killed himself leaving a note condemning Mr Clarke - who denies any wrongdoing.
Downing Street has stressed that Lord Feldman retains the "full confidence" of the Prime Minister.
But a senior Tory source confirmed the peer is among 40 "witnesses" who are giving written evidence to party officials.
Lord Feldman is understood to have launched the investigation into the allegations surrounding Mr Clarke in August, but has never played a part in it.
Former Tory MP Paul Goodman, editor of the influential ConservativeHome website, said the peer should go.
Writing for The Telegraph website, he said: "Before the weekend, the party's official view was that any action would be premature before the coroner's inquiry into Mr Johnson's death is complete - together with police inquiries that are also taking place. Mr Shapps's resignation has changed all that. If his position was untenable, it follows that Lord Feldman's is too."
The final straw for Mr Shapps appears to have been the emergence of an email sent to him by ex-minister Baroness Warsi in January, complaining that the election campaign aide had been abusing her on Twitter.
The party had previously insisted no complaints against Mr Clarke, who ran its RoadTrip canvassing drive in the run-up to the election, were received before August. He has since been expelled.
In a letter to the Prime Minsiter confirming his resignation as international development minister, Mr Shapps said he had concluded the "buck should stop with me".
He wrote: "Although neither the party nor I can find any record of written allegations of bullying, sexual abuse or blackmail made to the chairman's office prior to the election, I cannot help but feel that the steady stream of those who raised smaller, more nuanced, objections should have perhaps set alarm bells ringing sooner."
Law firm Clifford Chance LLP has been instructed to prepare a report on the issues raised, with a remit to assess whether complaints were handled properly and "identify any individuals who were at fault".
The firm's lawyers will also consider the integrity of the evidence-collecting process and whether the right people have been interviewed.
The collection of evidence is not expected to be completed until the end of the year, and Clifford Chance will produce the report "as soon as possible after that".
The source added: "It will be a decision of the board which parts of the report to publish having regard to protecting vulnerable witnesses, the ongoing coroner's inquiry and police investigation. Subject to this, the party intends to publish the key findings and recommendations."
Senior Tory backbencher Bernard Jenkin called on Mr Cameron to launch a "thoroughgoing" governance review of the party.
He said the party's board was too big and unwieldy, and there was a need to examine "the structures and the values by which we operate, the mission for the people who work and volunteer in the Conservative party and how we lead them, and what we expect acceptable behaviour and attitudes should be".
Tory MP Charles Walker, a member of the party board, told the Guardian today's meeting would have to consider a series of questions about the allegations.
"What we need to establish is this: did central office receive complaints about the conduct of Mark Clarke? If it did receive complaints, what was the nature of the complaints? How were the complaints recorded? And once recorded, how were they dealt with?
"If complaints were indeed received, we suspect they were, but we don't know, were the complaints leaked back to Mark Clarke? That would be deeply, deeply concerning.
"The whole thing is concerning. There is a lot of sound and fury about this but what has got to be established is what complaints were received by central office.
"Then there is a broader issue and that is around how young activists could possibly be led to believe that campaigning as a young Conservative is anything more than having fun with friends. The idea your performance as a young Conservative would have any bearing on your selection as a future parliamentarian is very worrying."