This will be the third complaint against the deputy prime minister over bullying allegations in recent weeks.
"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," the spokeswoman of UK Prime Minister Rishi Sunak was quoted by British media as saying.
The spokeswoman noted that the complaint was filed on November 23 and related to Raab's time as Brexit secretary in 2018, adding that an independent investigation was ongoing.
According to reports, the new complaint against Raab might also be damaging for Sunak, who has repeatedly reiterated support for his deputy prime minister and refused to dismiss him amid recent scandals.
Earlier in the month, Raab appealed to Sunak to begin an independent investigation into two formal complaints against him over alleged inappropriate behavior and bullying of employees.
On November 14, Simon McDonald, a former Foreign Office senior civil servant, told reporters that Raab was so rude to his employees during his term as foreign minister that people were "scared to go into his office."
On November 11, 15 employees of the Ministry of Justice were reportedly offered "respite or a route out" after Raab's reappointment, due to concerns over possible damage to their mental health.