Sir Ian Blair has been accused by one of his former top officers of compromising the Metropolitan Police's political independence.
Brian Paddick was formerly one of Sir Ian's top officers
The Met commissioner is too close to the Labour government, ex-deputy assistant commissioner Brian Paddick tells the Daily Telegraph.
He also said he felt forced to back plans to extend pre-charge detention of terror suspects to up to 90 days.
Mr Paddick is the Liberal Democrat candidate for mayor of London.
He claims the commissioner's political views have undermined public confidence in Scotland Yard's political neutrality.
"As soon as people perceive that the police commissioner is too close to a political party it undermines public confidence in the whole of the police force," he said.
"It calls the political independence of the police into question. Police officers want the public to be proud of them.
"If there's a perception that their chief is aligned to a political party that undermines the rank and file officers.
"His position is unsustainable. I think he should resign."
Labour's London Mayor, Ken Livingstone, backed Sir Ian, accusing the Met commissioner's critics of waging a "reckless and cynical campaign" against him.
He said: "Sir Ian Blair is one of the most successful Metropolitan police commissioners in years at reducing crime.
"The real record, which those calling for Sir Ian Blair to go wish to obscure, is that London now has record police numbers, neighbourhood policing in every part of London, and falling crime levels."
Sir Ian was personally criticised by the Independent Police Complaints Commission (IPCC) in its recent report into the death of Jean Charles de Menezes on 22 July, 2005.
The London Assembly has also issued a vote of no confidence in Sir Ian.
A week before the IPCC report was published, a jury convicted the Metropolitan Police of breaching health and safety laws when officers pursued Mr de Menezes to the Tube station and shot him seven times.
Mr Paddick used the launch of his mayoral campaign to align himself with the Lib Dems' call for Sir Ian to resign and said the commissioner had become a Labour ally.
He criticised Sir Ian for supporting controversial Labour proposals such as ID cards and increasing the 28-day detention period.
And he claimed he felt forced to put his name to a press release supporting former prime minister Tony Blair's failed attempt to introduce a 90-day detention limit.
"His [Sir Ian Blair's] office said 'you are a deputy assistant commissioner in the Metropolitan Police and as such you will support 90 days'. It felt as if I had a gun to my head."
A YouGov poll published in London's Evening Standard on Friday found that 44% of respondents thought Sir Ian should not lose his job as against 37% who believed he should.
The poll surveyed 1,039 people across London between 7 and 9 November.
Mr Livingstone said: "According to this polling the reckless and cynical campaign against Sir Ian Blair has failed to win over even four out of 10 of Londoners."