You can never quite tell with Boris Johnson whether what comes out of his mouth is planned or wholly accidental.

That he has some concerns about the coalition government’s planned changes to housing benefit is already known. But to put the changes, and their potential impact, into the same breath as ‘Kosovo-style social cleansing’ goes a long way beyond ‘some concerns.’

So whether it was planned or not – live radio has a habit of getting a combination of both – it will have been felt in Downing Street, that much is for sure … felt as in ‘ouch,’ as if David Cameron didn’t have enough on his plate today what with a very tricky European Summit.

At least when former Labour mayor Ken Livingstone fired broadsides at Tony Blair and Gordon Brown, there was nothing terribly surprising about them. They had always been on different sides of most arguments. But Johnson and Cameron, if no longer bosom buddies, are not enemies in the same way.

Take the Kosovo analogy to its logical conclusion, and Johnson is effectively putting his colleague and fellow Old Etonian/Bullingdon Club member into the same bracket as Slobodan Milosevic.

As I said yesterday, I am currently putting the finishing touches to volume two of my diaries, and Milosevic’s ethnic cleansing of Kosovo figures large towards the end of the book. He was odious, and his policies and their impact even more so. If I was Cameron, I would be pretty angry that the Kosovo analogy was made, much as I agree with his view that the policy could have a pernicious impact upon many London families.

If it was an accident, we simply add it to the list of over the top statements and gaffes from the gaffe-prone blundering Boris. If it was planned, then DC, we have a problem!