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
A new planet which could contain life has been discovered close enough to Earth to be reached by future space missions.
Crisis talks are being held over fears that trainee Catholic priests in Ireland are using the gay dating app Grindr.
Patience Chukwu was dragged along the street and punched repeatedly in the face near Homerton Hospital in east London.