Boris Johnson has been caught on film telling a black cab driver to "f*** off and die".
In a video obtained by the Sun and Daily Mail, the Mayor of London is seen riding his bike and being heckled by the driver of a Hackney carriage.
The Mail said the spat was over Mr Johnson's policy on rival minicab booking service Uber, which has led to several protests from London taxi drivers over unregulated competition.
In the 10-second video, captured by a bystander, the taxi driver can be seen leaning out of his window and shouting: "You are one of them mate, that's what you are. One of them."
The recently-elected Conservative MP then replies: "Why don't you f*** off and die - and not in that order."
As he drives away, the cab driver shouts: "Yeah b******s, hope you die."
The incident reportedly took place as Mr Johnson was cycling home on June 5 at around 12.20am in St John's Street, Islington, north London.
A source close to the mayor told the Mail: "I believe this particular cabbie is a serial heckler. He offered Boris a few choice words. Boris felt obliged to offer him a few in return. And off they went."
Steve McNamara, spokesman for the Licensed Taxi Drivers Association, told the paper: "The taxi drivers in London are rightly worried about the way private firms are operating. I can understand why the driver had a pop at Boris, and to be fair to him, he gave back as good as he got."