Labour leader Ed Miliband has described Boris Johnson as a "good showman but not a very good mayor".
Speaking to ITV London, Mr Miliband said that while the London Mayor was a "fun guy" it was not enough to be a good Mayor.
However, the Labour leader insisted comedian Eddie Izzard - who has announced his intention to stand as London Mayor - was welcome to stand for Labour in the 2020 contest.
More top news
Four QPR hooligans caught on CCTV reeking havoc and attacking rival fans have been jailed.
An armed gang who had just robbed a man in his home and were caught "within seconds" by police have been jailed.
The body of a 23-year-old who was last seen at the foot of Ben Nevis has been found following a large rescue mission.