Boris Johnson is said to have privately warned Dominic Raab about his behaviour when the latter held several cabinet positions under his leadership.
The former Tory PM is understood to have provided evidence to Adam Tolley KC, the lawyer investigating bullying allegations made by senior civil servants against Mr Raab.
Mr Johnson warned the senior Tory minister about his conduct while serving in his cabinet, according to The Telegraph. Both Mr Johnson and Mr Raab’s spokespeople declined to comment on the matter.
Multiple sources with knowledge of the investigation that the deputy prime minister had been questioned by the senior lawyer.
It has been one week since Mr Raab 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.
The eight formal complaints – involving at least 24 people – centre on Mr Raab’s tenures as foreign secretary, Brexit secretary and during his first stint as justice secretary.
Mr Raab has denied bullying and insisted he had “behaved professionally throughout” – but said that he would resign if an allegation of bullying was upheld.
“We believe innocent until proven guilty in this country”, he told Sky News, adding: “"If an allegation of bullying is upheld I will resign.”
Rishi Sunak has resisted calls to suspend his deputy while under investigation, including from ex-Tory party chairman Sir Jake Berry, Labour and the FDA union which represents senior civil servants.
If the investigation is 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 that Mr Sunak 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 Foreign Office’s former permanent secretary Simon McDonald has given evidence formally criticising Mr Raab, according to The Times.
Condemning his behaviour last year, Lord McDonald said Mr Raab was “as abrasive and controlling with junior ministers and senior officials as he was with his private secretaries.”
FDA general secretary Dave Penman said some staff who worked with the senior cabinet minister had suffered “mental health crises” and been forced to quit and downgrade jobs as a result of his behaviour.
The union boss also denied claims by Mr Raab’s allies that the complaints against him are a politically-motivated “conspiracy” to force him out as deputy prime minister.
Some witnesses to the inquiry have offered positive assessments of Mr Raab’s behaviour as well as the negative evidence about his conduct, it is understood.
Tory MPs told The Independent that was “a lot of frustration” about the Raab inquiry, who want Mr Sunak to hurry the investigation to a conclusion. “We want it to go away as quickly as possible,” one said.
During the investigation, Mr Tolley was expected to put all the allegations to Mr Raab before finalising his report.
