Downing Street was approached with claims of an alleged sexual assault by Sir Michael Fallon hours before he resigned, Sky News understands.
Sir Michael resigned as defence secretary on Wednesday night, admitting that his personal conduct had "fallen below the high standards we require of the armed forces".
It has now emerged that a third allegation was made against him within a week, after he apologised for putting his hand on journalist Julia Hartley-Brewer's knee during a Conservative party conference 15 years ago; and he was accused of making lewd remarks – which he denies – to cabinet colleague Andrew Leadsom.
The sexual assault claim was revealed to The Times by former minister Anna Soubry who said that a "person with great courage" had presented their allegation to No 10 on Wednesday, shortly before Sir Michael's resignation.
"Theresa May made it very clear she took these allegations seriously and within hours he was gone", said Conservative MP Ms Soubry who had served as Sir Michael's junior at the Ministry of Defence until the 2015 election.
Sir Michael issued a strong denial to the newspaper saying the claim was not true and libellous. He said: "I've already accepted that I have behaved inappropriately in the past but I have never assaulted anybody". He said no specific allegation was put to him.
Downing Street declined to comment this morning.
It comes amid a deepening rift in the Conservative party about the claims reportedly made by Mrs Leadsom, the Leader of the Commons. Ms Soubry said "somebody has betrayed Andrea Leadsom's confidence" by leaking the accusation.
But others in the party accused the former leadership contender of an act of revenge against Sir Michael after he is said to have suggested she would have to be sacked to get Cabinet agreement on a Brexit deal.
A minister told the Daily Telegraph: "She knifed him [Fallon] to make sure she couldn't be sacked because no-one is going to sack a whistleblower". Mrs Leadsom has not commented on the allegations.