Extracts from an infamous audio tape in which a youthful Boris Johnson discusses a plan to have a tabloid journalist beaten up are to be broadcast tomorrow for the first time.

Johnson, now mayor of London and chair of the Metropolitan Police Authority (MPA), can be heard agreeing to help his fellow Old Etonian, Darius Guppy, obtain the personal details of a News of the World journalist, Stuart Collier.

Collier had been asking questions about Guppy, who talked of hiring a contact from south London to assault the journalist.

The extracts will be aired in a Channel 4 Dispatches programme, The Trouble With Boris, which its producers say raises serious questions about the mayor.

During the 21-minute conversation, in the summer of 1990, Johnson, then a journalist on the Daily Telegraph, leads Guppy to believe that he would try to obtain Collier's home telephone number so that his home address could be traced.

Guppy, who declares he believes himself to be a "potential psychopath", gives "his word of honour" that Johnson's role in the assault will remain undetected.

When Johnson expresses concern about how severe the beating will be, Guppy tells his friend: "I guarantee you he will not be seriously hurt. He will not have a broken limb or broken arm, he will not be put into intensive care or anything like that. He will probably get a couple of black eyes and a ... a cracked rib or something." The revelation prompts Johnson to ask, "Cracked rib?" and Guppy to reply: "Nothing which you didn't suffer at rugby, OK? But he'll get scared."

On the tape, Johnson appears to be afraid his role in supplying the personal details will be found out and seeks reassurance from his friend that it will not. At the end of the conversation, Johnson is heard saying: "OK, Darrie, I said I'll do it and I'll do it. Don't worry."

A spokeswoman for the mayor's office did not return calls yesterday. However in a statement to Dispatches the mayor's office described the Guppy tape as a "colourful" tale from the past that bore no relevance to Johnson's current position. The mayor has always dismissed the tape as a joke.