Boris Johnson privately warned Dominic Raab about his behaviour when he was PM amid allegations of bullying, it is claimed.
Mr Johnson has reportedly provided evidence to the lawyer carrying out a probe into the Deputy Prime Minister's conduct.
Adam Tolley KC is investigating a string of claims made by civil servants who worked under Mr Raab, who had a number of Cabinet positions under Mr Johnson.
Mr Raab denies the claims, but Rishi Sunak faces calls to suspend him while the matter is looked into.
He said he will resign if an allegation of bullying is upheld.
Complainants have said they lost weight, suffered breakdowns or even felt suicidal as they themselves felt there was no escape, the Mirror revealed last month.
But Mr Raab denies the claims, saying he is “confident” he has “behaved professionally at all times”.
According to The Daily Telegraph, Mr Johnson "privately warned" him, and has taken the "highly unusual" step for a former Prime Minister to get involved in a Downing Street investigation.
It has been one week since Mr Raab was interviewed by Mr Tolley, indicating that the official inquiry could be drawing to a conclusion.
Dozens of witnesses including top departmental civil servants are believed to have spoken to the inquiry, which could determine Mr Raab's political fate.
It was unclear how many times he had been interviewed or when.
The investigation centres on when he was foreign secretary, Brexit secretary and during his first stint as justice secretary.
There are eight formal complaints involving at least 24 civil servants.
During the investigation, Mr Tolley would be expected to put the allegations to Mr Raab before finalising his report.
Former Tory party chairman Sir Jake Berry, the Labour Party and the FDA union, which represents senior civil servants, have piled pressure on Mr Sunak to suspend his deputy until the investigation reaches a conclusion.
If the findings are damning, Mr Sunak will again face questions about what he knew about the allegations before bringing Mr Raab back into the Cabinet.
Downing Street has only ruled out the Prime Minister being aware of "formal complaints", but sources say he had been warned about his ally's behaviour.
Mr Raab ordered the investigation in November after coming under pressure following numerous claims, including that he was so demeaning to junior colleagues that many were "scared" to enter his office.
The Mirror has contacted Mr Raab's office for comment.
