Boris Johnson has accused David Cameron of pretending not to know what Magna Carta means so he did not appear too clever.
But in a blow to his own carefully crafted man-of-the-people image, the London Mayor did not know who scored a hat-trick for England in the 1966 World Cup final.
Mr Johnson's claim the Tory leader feigned ignorance exposes the growing rift between the Prime Minister and the outspoken London Mayor, seen as his main rival to lead the Conservatives.
Mr Cameron was widely mocked for his appearance on American television's The Late Show with David Letterman earlier this week after the US host quizzed him on British history.
The grilling included being asked what Magna Carta meant and who composed Rule Britannia. The Prime Minister fluffed both questions, but his fellow Old Etonian and Oxford University contemporary Mr Johnson claimed Mr Cameron really knew Magna Carta meant Great Charter and faked the apparent knowledge gap to appear more down-to-earth.
Classics scholar Mr Johnson said: "I think he was only pretending. I think he knew full well what Magna Carta means. It was a brilliant move in order to show his demotic credentials and that he didn't have Latin bursting out of every orifice."
Quizzed on more modern British history during an appearance on LBC 97.3 radio, Mr Johnson failed to correctly identify Geoff Hurst as the scorer of a hat-trick in the 1966 World Cup final victory over West Germany, the only time England have ever been football world champions.
Mr Johnson claimed defender Bobby Moore netted three. In fact, Moore was the captain who held aloft the Jules Rimet trophy at Wembley. Perhaps realising the revelation could damage his dreams of claiming the keys to No 10 Downing Street, the 48-year-old mayor protested: "I was only two!"
In a wide-ranging interview and phone-in with presenter Nick Ferrari, Mr Johnson faced the anger of London taxi drivers frustrated by transport arrangements during the Olympics.
He apologised to businesses whose trade was hit by the summer spectacle, but denied claims the capital became a "ghost town". He said: "I'm not going to pretend it was a bonanza for everybody and for those that feel hard done by, I'm sorry for the losses or low takings they had. But the overall economic benefit to London of people around the world seeing a city that can deliver an amazing event like that so successfully was incredibly positive."