Boris Johnson was accused yesterday of undermining the investigation into Home Office leaks when he declared that he did not think there would be any charges.

The Mayor of London, who is also chairman of the Metropolitan Police Authority (MPA), said he had had an “instinct and a hunch” that the police investigation would “probably go very badly”. Mr Johnson also admitted speaking to Damian Green, the Tory MP, after his arrest.

Police chiefs and political opponents said that Mr Johnson’s failure to distinguish between his public responsibilities and political friends had left him vulnerable to a huge conflict of interest.

A senior Met source said that they were “dismayed” and “felt let down” by what Mr Johnson had said.

“Not only has he contacted a suspect, but he has given an opinion on an investigation without being aware of all the evidence. That completely undermines the officers involved,” the source said.

“If he has done this now, what is to stop him doing the same thing in the future on a bigger inquiry if he knows someone involved in it?”

Len Duvall, the leader of the Labour group on the London Assembly and a former chair of the MPA, said that Mr Johnson should consider his position as chairman of the MPA. “It is astonishing that the mayor, following a briefing from senior police officers, has been speaking to a suspect under police investigation,” he said.

“He received information as chairman of the MPA, then went to his political ally and old friend, who is now under criminal investigation. Then we have seen his view that they are not going to charge anyone and it is not going to reach the courts. It is just wrong, wrong for someone in his position to be saying that.”

Mr Duvall added that he was considering referring the matter to the Committee on Standards in Public Life, an independent body that can make recommendations for individuals to be removed from office.

Mr Johnson’s comments came during what should have been a routine meeting of the London Assembly. They risked damaging relations with Sir Paul Stephenson, interim replacement at the Met after the mayor forced the resignation of Sir Ian Blair as commissioner.

Seated beside a stony-faced Sir Paul, Mr Johnson said: “Obviously we are awaiting the outcome of the inquiry, but I am yet to be persuaded that it is likely to lead to charges or successful prosecutions.”

Mr Johnson added: “There has been a huge amount of political hoo-ha about this, but there are also lots of people out there who would think it’s absolutely right that the police should get on with their job. That’s quite right, but my own feeling is that these political actions could easily be construed as disproportionate.”

At the beginning of the meeting Sir Paul read out a statement defending his officers. He said at issue was the ability of the Met to maintain operational independence and he said that everyone involved “must tread carefully and with caution” when speaking about the investigation.

Despite Mr Johnson’s comments, police said that they were determined to carry on with the investigation and would not be swayed by political interference.

A police source said: “This reinforces what Sir Ian Blair believed – that the Met is being used as a political football.”

Times Recommends

• Sarah Brown says 'no thanks' to veal at G8 - twice
• Stockton-on-Tees is obesity capital of Britain
• Stores blame poor sales on Murray factor