The Deputy Prime Minister is facing fresh bullying allegations from civil servants across several government departments.
This week, the Government appointed lawyer Adam Tolley to investigate two complaints about Mr Raab from staff at the Ministry of Justice and the Foreign Office.
A third, relating to his time as Brexit Secretary in 2018, is now being looked into as part of the bullying inquiry.
A spokesman for Prime Minister Rishi Sunak said on Friday that the Cabinet Office received the new allegation on November 23.
“It was looked at as you'd expect, the Prime Minister was provided with advice and then he asked for the scope [of the investigation] to be updated,” they said.
“I can confirm that the Prime Minister has now asked the investigator to add a further formal complaint relating to conduct at the Department for Exiting the European Union and to establish the facts in line with the existing terms of reference.”
Mr Raab, who is also the Justice Secretary, requested the investigation into his conduct towards staff in the wake of two earlier complaints.
He denies the allegations.
Downing Street said Mr Tolley's report will be published "in a timely way".
Any final judgement on whether Mr Raab was in breach of the ministerial code will remain Rishi Sunak.