Dominic Raab is facing fresh questions over his conduct as several of his former staff are said to be preparing to file formal bullying complaints against him.
The move is a co-ordinated effort by “a raft of senior civil servants in multiple government departments” to lodge concerns over Mr Raab’s behaviour, including a number of private secretaries, BBC Newsnight reported.
Meanwhile, the programme said it had heard allegations that the Cabinet minister used his personal email account for Government business at two different departments, as recently as last year.
It comes after senior lawyer Adam Tolley KC was appointed by Rishi Sunak to look into two formal complaints of bullying by the Deputy Prime Minister.
But the terms of reference for his inquiry listed only the two formal complaints made so far.
Labour said its scope must be expanded to ensure there is “no hint of a whitewash”.
Mr Raab, who returned as Justice Secretary when the Prime Minister took office, requested the independent investigation and said he would engage “thoroughly”.
He has said he is “confident” he has behaved “professionally” throughout.
Dave Penman, the general secretary of the FDA union which represents civil servants, claimed the Prime Minister was briefed about Mr Raab’s behaviour before he appointed him.
Mr Sunak previously said he had not been aware of any “formal complaints” about the Cabinet minister’s conduct.
Mr Penman said the PM needs to make a call on whether it is “safe” for his deputy to continue in his role while the investigation is ongoing.
“Dominic Raab, as Deputy Prime Minister and the Secretary of State at the Ministry of Justice, is still dealing with civil servants just now,” he told Newsnight.
“So if there are serious allegations about his conduct that the Prime Minister has seen, he has to make a decision, is it safe, essentially, for civil servants to continue to work with him. And that’s what any employer would do.”
Mr Tolley was appointed by the Prime Minister and will report to him.
The terms of reference require Mr Tolley to “establish the specific facts” surrounding the formal complaints made against Mr Raab at the Ministry of Justice and the Foreign Office.
They specify “the investigation should be completed as swiftly as possible”.
Downing Street said the report into Mr Raab will be published in full but would not put a timeframe on the inquiry, saying only it would be detailed “in a timely way”.
The Prime Minister’s official spokesman said the lawyer had been appointed to “conduct the investigation, establish the facts and provide his findings”.
“He’s asked to look into those two formal complaints to establish the facts. He’s also able to discuss with the Prime Minister if he wants to look into other issues that may arise,” he said.
In an indication that Mr Tolley may be able to interview people and get records of WhatsApp messages and emails, the spokesman said: “He will have access to all the information he wishes to see.”
“The scope of this investigation must immediately be expanded to enable proactive investigation of Dominic Raab’s behaviour during his time as a minister, including so-called expressions of concern, informal complaints and the concerning testimony of his own former permanent secretary.
“This Conservative Government has a troubling track record of brushing serious misconduct under the carpet. Their refusal to act on findings against Priti Patel previously led their former ethics chief to quit in disgust.
“A temporary stop-gap investigator, appointed in a panic, with an absurdly narrow remit is not a solution to dealing with the flood of allegations of ministerial misconduct now requiring investigation.”
The appointment of Mr Tolley to investigate the claims involving Mr Raab was necessary because there is not a permanent ministerial ethics watchdog.
Lord Geidt quit as the independent adviser on ministers’ interests in June and has not yet been replaced.
Mr Tolley’s legal career has seen him involved in high-profile cases including successfully representing Clarence House in a 2004 employment tribunal brought by aide Elaine Day who accused the then Prince of Wales’ household of sexual harassment and unfair dismissal.
He also acted for HM Revenue and Customs in a case with TV presenter Eamonn Holmes over off-payroll working arrangements.
Liberal Democrat chief whip Wendy Chamberlain MP said if the complaints against Mr Raab were upheld the Prime Minister must sack him as “anything less would make a mockery of his promise to bring back integrity”.