Scotland Yard's race row escalated on Thursday as three officers were suspended over allegations of offensive comments to colleagues.
The Independent Police Complaints Commission announced it was formally investigating two PCs and an acting sergeant.
The police watchdog was contacted by the Metropolitan force on Wednesday - more than two weeks after the complaint was made about their conduct between January and March.
Fresh allegations come after Commissioner Bernard Hogan-Howe vowed to get to the bottom of "very damaging" footage of one officer apparently racially abusing a man and another allegedly assaulting a teenage boy last summer.
More to follow.