Boris turning to dark side, says Ken as fares battle gets abusive

Mr Livingstone said the Mayor was "lashing out" and "flailing around". Mr Johnson, who is at the World Economic Forum in Davos, had attacked his rival's pledge to slash transport fares by seven per cent, calling it "a lie".

The Mayor said that Mr Livingstone "has lied twice in the past about fares" and warned Londoners: "You can't trust Ken."

Experts today warned that Londoners do not want to see an "abusive campaign" dominated by the two candidates sniping and mounting personal attacks.

In a letter to the Mayor today, Mr Livingstone wrote: "Far from being the re-coronation you once hoped this contest would be, it's now accepted that the race will be closely fought... but no one could have expected that you would react so badly to a little bit of pressure. Suddenly the Conservative party in London is flailing around and lashing out."

He added: "It has not taken much for your party to turn to the dark side."

Mr Livingstone accused Mr Johnson's campaign staff of making "false" claims that he would consider reversing the alcohol ban on public transport if elected and that his running mate, Assembly member Val Shawcross, has called for a council tax rise.

The Back Boris campaign today said that Mr Livingstone attacked the alcohol ban during a recent speech to students at Queen Mary University and that the Labour Group at the London Assembly had mentioned a council tax rise in a recent press release.

Mr Livingstone's letter said: "Come on Boris, this isn't you. You no longer seem to be speaking for yourself. Where has the Boris Johnson who could always be relied upon to make us smile (if not govern competently) gone? "Let's try to keep this election clean."

Mr Johnson's campaign team reacted furiously. A spokesman campaign said: "Our focus on policy is in sharp contrast to Ken Livingstone's personal attacks, including likening Boris Johnson to Hitler and calling his chief of staff a war criminal."

Tony Travers, of the London School of Economics, said: "Politicians like trading abuse but the public never likes it. Clearly people are not going to make a decision in the mayoral election on the basis of who is more abusive."

A ComRes poll this week put Mr Livingstone on a 51 to 49 lead for the May 3 poll.