Boris Johnson is understood to have privately warned deputy prime minister Dominic Raab about his conduct.
Mr Raab is facing a bullying investigation following several complaints about his behaviour by civil servants.
Former prime minister Mr Johnson, who first appointed Mr Raab as deputy PM, is understood to have provided evidence to Adam Tolley KC, the independent lawyer who is carrying out the investigation, The Telegraph reported.
It is very unusual for a former prime minister to be involved in an investigation launched by Downing Street.
Mr Raab, who is deputy PM and justice secretary, was interviewed by Mr Tolley a week ago, indicating the official inquiry could be coming to an end.
Sky News has been told the inquiry is drawing to a close but the report will not be released imminently.
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Mr Johnson and Mr Raab have both declined to comment on the latest claim.
A spokesman for the former PM told Sky News: “It would be inappropriate to comment on an ongoing inquiry.”
The inquiry is believed to have spoken to dozens of witnesses, including top civil servants.
Claims have been made that Mr Raab created a “culture of fear” at the Ministry of Justice and he was “very rude and aggressive”.
Colleagues were allegedly “scared” to go into his office when he was foreign secretary, former permanent secretary Lord McDonald has said.
Mr Raab has denied he has bullied anybody, telling Sky News last month he had “behaved professionally throughout”.
He also pointed out he called the inquiry into himself when the accusations were made.
The claims against Dominic Raab
The deputy PM has said he would resign if the inquiry found he had bullied staff.
“Look, if an allegation of bullying is upheld, I will resign,” he told Sky News’ Sophy Ridge on Sunday programme last month.
Rishi Sunak has faced calls to suspend Mr Raab while the investigation is carried out but he has said the inquiry should run its course before he makes a decision.
If Mr Raab is found to have bullied staff, Mr Sunak will face questions about what he knew about the allegations before giving Mr Raab his role.
There are eight formal complaints against Mr Raab based on his time as foreign secretary, Brexit secretary and during his stint as justice secretary under Mr Johnson.