The off-duty inspector is alleged to have racially abused a council parking enforcement officer in Barnet, North London.
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Channel 4 News understands the officer in question is Detective Inspector Tony Brown based in Enfield, who took charge of investigations into last August's riots in the borough.
It is not known what racially abusive language was allegedly used, but the inspector is under suspicion of committing an offence under the Public Order Act 1986. It is the same law which has been used to charge a police constable in Newham with racial abuse for allegedly using the 'n' word when arresting a black student.
In a statement, Commander Mak Chishty said: "The people of Enfield rightly expect their police officers to demonstrate the highest standards of professionalism at all times.
"Any allegation of racism will be investigated thoroughly and appropriate action taken."
The Independent Police Complaints Commission says DI Brown is the highest-ranked officer to be investigated since the Yard began submitting all public complaints alleging racism to the IPCC for examination.
Barnet Council which employs the wardens said the authority took any allegations of abusive behaviour directed towards its staff very seriously....and was awaiting the outcome of the police investigation.
The Commission is now dealing with a whole raft of investigations:
20 cases have been referred since 1 April, when it was announced the force would inspect every complaint of racism
8 investigations are being supervised by IPCC
9 have been sent back to the met for internal investigation
3 investigations are being carried out by the IPCC's own investigators