Boris Johnson was today at the centre of a conflict of interest dispute after he admitted speaking to arrested Tory MP Damian Green and being convinced of his innocence.

The Mayor, who is also self-appointed chair of the Metropolitan Police Authority, was accused of pre-judging the police investigation into Whitehall leaks.

He denied passing on any sensitive operational information to his friend and former colleague, who he has spoken to since his arrest, but said he was simply trying to establish whether police action was appropriate. Scotland Yard has been struggling to justify its actions since it held Mr Green and searched his London and constituency homes as well as his Commons office last week.

But acting Metropolitan Police Commissioner Sir Paul Stephenson robustly defended his force's actions at the regular London Assembly meeting.

He insisted the police had an obligation to follow up evidence "wherever it may take us" and denied there had been any political interference from either the Mayor or Home Secretary Jacqui Smith during the inquiry.

Sir Paul also explained that counter-terror officers had arrested Mr Green because the Met's anti-terror operations had merged with Special Branch, responsible for investigating potential breaches of national security, in 2006.

He denied the police had used Home Office civil servant Christopher Galley, who was arrested last month, to help entrap the MP or that they had used any electronic tapping devices.

Mr Johnson was first informed of the Mr Green's arrest shortly before it took place last Thursday. At the time, he warned the acting Met chief that he felt the action was disproportionate and would be deeply controversial.

"I think it was entirely right as chairman of the MPA to give my instinct, my feeling, my hunch, that I thought this would probably go very badly and that people would then take a lot of convincing that this action was not disproportionate," he told the Assembly today.

"For my own part, of course we have to await the outcome of the inquiry, but I still remain to be convinced that it wasn't disproportionate. So I abide by what I said."

But London Assembly members were astonished by his decision to contact Mr Green - a suspect in an ongoing investigation.

Labour member Len Duvall, former chair of the MPA, said that the row should prompt the Mayor to consider his position as chairman of the Met watchdog. He said: "I'm absolutely astonished and it's completely wrong in terms of his position."

Mr Johnson justified his decision saying it was "perfectly proper" to try to establish the full facts of the case.

He admitted the MP was "a friend and an ex-colleague" and that he was concerned when he heard the police search of his home had made Mr Green's children cry. "I'm not involved in an investigation. I'm not in any way a part of the operational side of this. I think there's a duty to inform myself about what is going on to avoid unnecessary errors," Mr Johnson said.

"I thought I did the right thing there for the sake of my own conscience, just to talk to Damian and make sure that I've understood circumstances correctly and I see absolutely no harm in that whatsoever.

"I cannot possibly be perceived of straying into operational territory. I'm a mere toenail in all of this."

He also stunned Assembly members by apparently prejudging the outcome of the police investigation into the alleged leaks. "It's my hunch that it will not produce either a charge or a prosecution," he said.

Dee Doocey, the Liberal Democrat London Assembly spokeswoman on policing, said: "I find it frankly astonishing that Boris Johnson told the Assembly that 'it is unlikely that any charges or prosecution will be brought'.It is not the job of the MPA chair to make public his views on the outcome of an on-going investigation. Boris Johnson should now reflect if he is the right person to chair the authority at this time."

But shadow foreign secretary William Hague sprung to the defence of the Mayor. "Boris Johnson was until recently a Member of Parliament. Like all of us, he would have contacts with his colleagues. You can't expect Boris Johnson not to know other politicians so I can't see that there's a conflict of interest there," he said.

When asked by Sky News if Mr Johnson had been wrong to reveal details of an active police investigation, Mr Hague said: "No. I think Boris is entitled to speak out, after all he replaced a Mayor who spoke about everything he ever saw fit to speak about." But Labour Assembly member John Biggs earlier told the Mayor: "The public perception is that if you're speaking to someone who could become an accused person, a witness, a suspect in a criminal matter and you are chair of the police authority, that there is a conflict in the public mind about your two roles there."

Meanwhile, Sir Paul insisted the decision to arrest Mr Green - and to search his homes and Commons office - had been taken by police in line with normal operational procedures.

He denied that either the Home Secretary or the Mayor had interfered in the investigation but said Ms Smith had sought his assurances that the inquiry was lawful and necessary He added: "It's right that we should be held to account at the appropriate time."

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