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Labour's London chief today threw his weight behind Tessa Jowell for Mayor, saying he wanted her “in the trenches” with him in the battle for City Hall.

Len Duvall, the party’s regional chair, said she was the only candidate who knew what levers to pull to get things done, and who would be able to pick up enough second preference votes to win the capital.

“I don’t agree with everything Tessa has done but if I was going into battle I would want her to be in the trenches with me,” he said.

“If the key is around some of the inequalities that exist then she is the front-runner for me because she understands what the levers are.

“People have got to think carefully about any other candidate in terms of their skills and also how they would attract those second preference votes.”

A public endorsement from the party chair, who was on the panel that drew up the short list, is unusual but possible because of the one member, one vote system in place to pick the candidate.

It comes as 300 London councillors publicly agreed to back Dame Tessa, representing 225 of the 628 different wards in the capital.

Mr Duvall warned the race would not be easy. “There’s weeks to unfold, I don’t think it’s a shoe-in for any candidate,” he said.

“There’s lots of myths about London and every election we fight here is a tough battle. I’ve never bought into this is a London Labour city, two victories by Boris Johnson have told me this.

“I’m not complacent about this. It doesn’t matter which candidate wins, you can’t take people for granted, you can’t live on past results, you have to work hard and have the right offer.”

The London Assembly member warned it would be "extremely damaging" to the party's chances if candidates used group votes to get selected when the process was supposed to be one member, one vote.

Asked whether candidates could be recruiting groups of union members and other interested parties, including from mosques, he replied: "Every candidate will be encouraging their supporters to join as registered supporters. The party will be monitoring this process closely.

"My understanding is that everyone should be individually registered.... There should be no group joining to minimise those abuses."

Mr Duvall said it was up to individual candidates not to exert any undue pressure on any group to sign up.

"The responsibility is on the candidates to enter into the selection in the spirit in which the party set it up. It would be extremely damaging if they didn't".

A spokesman for Sadiq Khan, said: "It is deeply offensive to all Londoners that Len Duvall has singled out the Muslim community in his endorsement of Tessa Jowell. We would hope that Tessa distances herself from these comments immediately. You can't become Mayor by dividing London and Londoners."