Rishi Sunak has asked for the probe into alleged bullying by Dominic Raab to beryllium expanded to see a 3rd ceremonial complaint.
Downing Street says the ailment related to Mr Raab's behaviour erstwhile helium was Brexit caput successful 2018.
Earlier this week, No 10 confirmed that a lawyer would analyse 2 complaints astir Mr Raab.
Mr Raab, who is some the justness caput and lawman premier minister, has denied immoderate allegations of bullying.
He has insisted helium "behaved professionally astatine each times" and that helium looks guardant to dealing with the complaints "transparently alternatively than dealing with anonymous comments successful the media".
Mr Raab is simply a adjacent state of Rishi Sunak and Downing Street has said helium has the PM's afloat confidence.
The determination to grow the enquiry comes aft sources told the BBC the Ministry of Justice, wherever Mr Raab present works, has been "inundated" with complaints of alleged bullying.
Last week, Mr Raab asked Mr Sunak to motorboat an enquiry into his behaviour aft allegations astir his behaviour towards staff.
The authorities appointed lawyer Adam Tolley KC to "establish the circumstantial facts" astir 2 ceremonial complaints that person been lodged astir Mr Raab's behaviour erstwhile helium was overseas caput and justness secretary, during Boris Johnson's premiership.
The lawyer volition study to Mr Sunak, who volition marque the last judgement connected whether Mr Raab's behaviour breached the ministerial code.
But the scope of the enquiry tin beryllium widened "at the discretion of the premier minister, successful consultation with the investigator," according to its presumption of reference.
Mr Raab was sacked arsenic justness caput and lawman premier curate by erstwhile PM Liz Truss erstwhile she took powerfulness successful September.
But the Esher and Walton MP was reappointed to some roles by Mr Sunak pursuing his predetermination arsenic Conservative person by the party's MPs.
There is present a coordinated effort by erstwhile backstage secretaries of Mr Raab to guarantee their allegations are heard arsenic portion of the investigation.
Private secretaries enactment successful the backstage bureau of authorities ministers connected the day-to-day moving of the department, including managing the minister's diary and advising connected argumentation matters.
The BBC has been told that a fig of these civilian servants crossed aggregate departments are preparing to taxable their ceremonial complaints.
Of these complaints, immoderate person already been submitted formally to authorities departments, portion others are being readied.
They screen a wide timescale, spanning from erstwhile Mr Raab was a inferior curate to his clip successful Cabinet, which helium joined successful July 2018 arsenic Brexit secretary.
Labour's lawman person Angela Rayner said: "There indispensable beryllium nary hint of a whitewash erstwhile it comes to the slew of superior allegations the lawman premier curate present faces."
Separately, BBC Newsnight has besides been told that Mr Raab received aggregate warnings from officials not to usage his idiosyncratic email relationship for authorities business.
Last month, Home Secretary Suella Braverman was forced to discontinue aft sharing authorities documents utilizing her backstage email.
However, Mr Raab has argued his usage of backstage emails did not breach the rules, and that the ministerial codification allows for it to beryllium done successful immoderate circumstances.
"I've ever taken proposal connected the close means, peculiarly having been overseas caput and dealing with a full scope of delicate issues, I've ever been precise cautious to support the integrity of immoderate communications I had," helium said.
Asked if helium had been warned by civilian servants not to usage his ain telephone for authorities business, helium replied: "No."
A person of the curate has told the BBC helium utilized a backstage relationship connected occasions for approving tweets and quotes related to authorities business.
Liberal Democrats Chief Whip Wendy Chamberlain said the Cabinet Office should "determine instantly if overseas enemies could person seen nationalist secrets sent by Dominic Raab".
A Downing Street spokesperson said: "Ministers are capable to usage assorted forms of communication. As agelong arsenic they instrumentality heed of that guidance, determination is not a binary regularisation connected usage of idiosyncratic email addresses."