Scotland Yard's former head admitted today that his force developed a "defensive mindset" that stopped it properly assessing whether to reopen its original phone hacking investigation.
Sir Paul Stephenson, who quit as Metropolitan Police commissioner over the hacking scandal, told the Leveson Inquiry into press standards that his officers failed to "challenge" assumptions about the 2006 probe.
Inquiry chairman Lord Justice Leveson suggested that police carried out a "back-of-the-envelope" review when the Guardian revealed in a July 2009 article that the illegal practice was far more widespread than previously believed.
Sir Paul also revealed that London Mayor Boris Johnson's deputy, Kit Malthouse, complained about the high level of resources the Met devoted to the new phone hacking investigation it finally launched in January 2011.
Scotland Yard's original hacking inquiry resulted in the jailing of News of the World royal editor Clive Goodman and private investigator Glenn Mulcaire in January 2007 after they admitted intercepting voicemail messages left on royal aides' phones.
But the force was widely criticised for limiting the scope of the investigation despite evidence from Mulcaire's notebooks that there could be many more hacking victims.
Sir Paul told the Leveson Inquiry he did not read the July 9 2009 Guardian story, which claimed his force's original hacking probe was inadequate.
Instead he told then-assistant commissioner John Yates to look into the newspaper's allegations.
The former commissioner said: "It was just yet another headline - I don't mean to say it dismissively - some noise about an event that I expected someone to pick up and deal with."
Mr Yates began his examination of the Guardian's claims on the morning the article appeared, and in the late afternoon made a media statement dismissing calls for the phone-hacking investigation to be re-opened.
Lord Justice Leveson asked Sir Paul: "Do you think in 2009 it was a reasonable approach to respond to what was this very detailed, researched article - which I appreciate you hadn't read - by what perhaps I might be forgiven for describing as a back-of-the-envelope job for the day, and coming out so quickly with a response?"
The former commissioner replied: "My understanding is there was much ongoing work after that date to continue to consider was there anything new coming to light, but that's a matter that only Mr Yates can have the discussion with you (about)."
Sir Paul said he had further discussions with Mr Yates when the New York Times published a lengthy article in September 2010 about phone hacking at the News of the World.
He told the inquiry: "I would have challenged him to say, 'are you absolutely sure we shouldn't open this up any further?'
"I was satisfied with the briefings I was getting and I'm as good as the briefings I get."
The former commissioner said the Met developed a "defensive mindset" regarding phone hacking very early, adding: "That stopped us going back and challenging what was the reason for the original investigation."
He went on: "We got ourselves almost hooked on a defensive strategy that we wouldn't expend significant resources without new or additional evidence."
Sir Paul resigned from the Met last July after facing criticism for hiring ex-News of the World executive editor Neil Wallis as a PR consultant and for accepting free accommodation at a luxury health spa worth thousands of pounds.
He told the inquiry he stood down as Britain's top policeman out of a "sense of duty and honour" but might not have quit had it not been for his ill-health.
The former commissioner was off work for four months until April 2011 after undergoing emergency surgery to remove a pre-cancerous tumour from his femur and a second operation to repair a fracture caused by the growth.
He said he was reluctant to take up the offer of a free stay at a Champneys health resort from the owner Steven Purdue, a close friend of his daughter's father-in-law.
But he accepted it to help his rehabilitation so he could get back to work at New Scotland Yard as quickly as possible.
Home Secretary Theresa May and Mr Johnson both opposed Sir Paul's resignation when he told them he was going to quit, the inquiry heard.
The inquiry heard that Mr Malthouse, the head of Scotland Yard's former governing body the Metropolitan Police Authority (MPA), raised concerns about the resources allocated to the Met's new hacking investigation, called Operation Weeting.
Sir Paul said in a statement: "On several occasions after Operation Weeting had started and I had returned from sick leave, the chair of the MPA, Kit Malthouse, expressed a view that we should not be devoting this level of resources to the phone hacking inquiry as a consequence of a largely political and media-driven 'level of hysteria'."
Commenting on Mr Malthouse's comments, the former commissioner said: "The reality was that this was wrong but that was a fairly widely held view."
Labour MP Chris Bryant, who was a victim of phone hacking, called for Mr Malthouse to lose his job.
He said: "This amounts to a clear political intervention designed to intimidate the Met into dropping an investigation...
"In any other country this kind of political manipulation would be considered wholly unacceptable and corrupt.
"It is no longer possible for Londoners to have confidence in the Met with Kit Malthouse sitting at the top table. Kit Malthouse should either resign or Boris Johnson should be forced to sack him."
The Leveson Inquiry, sitting at the Royal Courts of Justice in central London, will hear evidence tomorrow from former Met commissioners Lord Condon and Lord Stevens.