A senior lawyer is investigating eight complaints of bullying against the deputy prime minister.
Deputy Prime Minister Dominic Raab has said he would resign if an inquiry finds he has bullied civil servants.
Eight formal complaints have been made against Mr Raab, who was appointed deputy prime minister and justice secretary last October.
Adam Tolley KC has been investigating the allegations after being appointed by the prime minister in November.
Mr Raab denies bullying, but told Sky News “if an allegation of bullying is upheld, I would resign”.
The bullying complaints relate to Mr Raab’s previous periods as justice secretary and foreign secretary under Boris Johnson, and his time as Brexit secretary under Theresa May.
At least three senior civil servants who worked with Mr Raab have given evidence to the inquiry into his behaviour as witnesses.
The BBC has found that other civil servants who allegedly planned to file complaints did not after learning they would have been identified to Mr Raab as part of Mr Tolley’s inquiry.
Both Labour and the Liberal Democrats have called for Rishi Sunak to suspend Mr Raab during Mr Tolley’s investigation.
The prime minister has said he will wait for the outcome of the inquiry before taking any action.
In November, Mr Sunak repeatedly declined to say whether he had informal warnings about Mr Raab’s behaviour before bringing him back into government.
Speaking on the BBC’s Sunday with Laura Kuenssberg, Mr Raab said: “I am confident I behaved professionally throughout.”
But he added it was right for ministers to “challenge assumptions and test ideas” when working with civil servants.
The union which represents civil servants, the FDA, found one in six civil servants had seen unacceptable workplace behaviour by a minister in the past year.
The FDA’s annual survey of senior civil servants also found 69.3% of respondents said they had no confidence in the current complaints system.
Mr Tolley is not expected to report his findings for several weeks and the prime minister will decide the justice secretary’s political future when the investigation concludes.
Privately, many Conservative MPs, including ministers, have told the BBC they fear the allegations could yet cost Mr Raab his job.
Mr Raab was justice secretary and deputy prime minister when Boris Johnson was succeeded by Liz Truss.
She sacked him, but he was reappointed to those roles when Mr Sunak entered Downing Street in October.
Mr Raab previously served in the cabinet as foreign secretary from 2020-21 and Brexit secretary in 2018.