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.
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Bus strikes are planned for Monday 29 December and Monday 5 January.