Dominic Raab will reportedly face fresh complaints about his behaviour from more senior civil servants across several government departments.
Mr Raab, who was reappointed as Justice Secretary and Deputy Prime Minister by Rishi Sunak in October, has been the subject of multiple allegations around his behaviour while at the Foreign Office and during his first stint in the justice brief.
The Prime Minister last week accepted Mr Raab's request for an investigation, which is being led by senior lawyer Adam Tolley KC, into two separate formal complaints lodged about his behaviour.
A number of Mr Raab's former private secretaries, who handled his day-to-day affairs while in different government briefs, are now preparing to submit formal complaints, BBC Newsnight reported.
The programme said it had heard accusations that the Cabinet minister used his personal email account for government business at two different departments and as recently as 2021.
Dave Penman, the general secretary at the FDA trade union which represents civil servants, told the BBC: "There are a series of allegations that appear to be coming across a number of departments where Dominic Raab has worked.
"Each one is going to have to be approved by the Prime Minister for investigation.
"This is an extraordinary set of circumstances. We've never come across a situation where so many civil servants appear to be raising complaints about a minister's conduct."
The allegations of bullying have all been denied by Mr Raab, who told MPs last week he would "rebut and refute" them.
Civil servants working at the Ministry of Justice were reported earlier in November to have been offered a "route out" when they learned he had rejoined the department.
This was followed by a number of further claims about his behaviour towards staff, although the current terms of reference only include the two formal complaints that have been lodged to date.
The Prime Minister's official spokesman insisted on Thursday lunchtime that Mr Sunak still had "full confidence" in his deputy.
Asked whether Mr Tolley's work would expand to consider new allegations about Mr Raab, the spokesman said: "The terms of reference make clear about how the scope of the investigation can change following discussions with the investigator and the Prime Minister.
"The investigator looking into this is highly experienced and has suitable background, and he's been given access to whatever they need to conduct a thorough and swift investigation."
