The Tory frontrunner savaged Mr Khan out of nowhere, branding him “useless” and “invertebrate” and “not a patch on the old guy.” The onslaught was woven into Mr Johnson’s wider solution to a question that had been posed on monocultural housing policies. A member of the audience asked the former London Mayor: “How will you ensure the Government's housing policies don’t lend themselves into creating ethnic categories inadvertently?”

Without hesitation, Mr Johnson blasted: “You build fantastic housing in the right place.

“And you put in superb transport infrastructure so you can create mixed communities where there are high quality jobs.

“And if you look at the disasters of planning in the ‘60s and ‘70s where monocultural estates were built, it’s because there simply wasn’t the transport infrastructure.

“Look around London and look at the estates outside London - you can see exactly what went wrong.”