Pressure is continuing for an inquiry into claims anti-terror intelligence leaks had put lives at risk.
Deputy Assistant Commissioner Peter Clarke described as "beneath contempt" those who leaked details to the media.
Both government and police figures have come under suspicion. Tory leader David Cameron said a Whitehall inquiry was needed - but Tony Blair rejected that.
Commons leader Jack Straw said a police inquiry would be appropriate if anyone had "prima facie" evidence of a leak.
On Tuesday, DAC Clarke, the UK's counter terrorism chief, said there were a "small number of misguided individuals who betray confidences".
He did not specify where the leaks were coming from, but suggested culprits were trying to "squeeze out some short-term presentational advantage" through secret briefings.
In the Commons on Thursday, shadow commons leader Theresa May asked why, when newspaper reports suggested the source of the leak was the home secretary's special adviser, the government was refusing an independent inquiry.
Mr Straw said her allegations were "wholly unsubstantiated", but said the claims that there had been leaks were serious, and could interfere in a criminal inquiry.
The truth is we can't find out unless we have a leak inquiry and that's why we need to have one
He added: "If anybody has prima facie evidence in the respect of a leak.... and I mean prima facie evidence, I don't just mean newspaper reports, it's a matter for a police inquiry."
Other reports suggest the leaks might have come from Scotland Yard itself.
The Lib Dems have written to West Midlands police to ask them to confirm they will be investigating the leaks. Home Affairs spokesman Nick Clegg told the BBC the Official Secrets Act may have been broken.
The Conservatives have asked Cabinet Secretary Sir Gus O'Donnell to launch a separate inquiry.
On Wednesday Mr Cameron's calls were rebuffed by the prime minister, who said "as far as I'm aware", no minister, civil servant or special adviser had leaked information.
On Thursday Mr Cameron told the BBC: "The truth is we can't find out unless we have a leak inquiry and that's why we need to have one."
The BBC has learnt that Home Secretary John Reid has already denied that his special advisers had anything to do with leaks of police anti-terror intelligence over the Birmingham raids in January.
Liberty director Shami Chakrabarti wrote to Mr Reid about the issue in February, shortly after reading about police concerns their operation was being undermined. She told the BBC she had got "a completely categorical denial".
Liberty has also put in Freedom of Information requests on the issue and also about what guidance was being given about handling requests for information on "ongoing operations".
The Home Office has asked for two extensions to reply, but has said it will do so by 3 May.
Labour MP for Birmingham Sparkbrook and Small Heath Roger Godsiff said there should be an internal inquiry to establish whether anyone in the police was responsible for the leak.
"If it is subsequently found that they had abused their position, there should be criminal charges preferred," he told the programme.
He said the same went for any Home Office officials, if they were involved.
Meanwhile, it emerged that the home secretary, John Reid, had denied earlier this year that officials from his department were involved in the leaks.
The Guardian has learned that journalists received up to three separate briefings from officials and police about allegations a group of men were planning to abduct and behead a Muslim British soldier.
One of the officials who leaked information was from the Home Office, the Guardian has been told.
In a letter to Shami Chakrabarti of Liberty, the human rights pressure group, in February, the home secretary wrote: "I can ... assure you that my special advisers did not brief the media on operational matters on or off the record, and that it is not their practice to do so."
In the Commons today, Mr Straw said that he "wholly deprecated and deplored" such leaks, but argued that the allegations potentially involved interference with a criminal investigation and were therefore a matter for police.
Referring to the Guardian's front-page story today, the shadow leader of the Commons, Theresa May, said: "Contrary to assurances given by the home secretary to the shadow attorney general, it is reported today that the source of the leak is the home secretary's special adviser.
"Why is the government refusing an independent inquiry?"
For the Liberal Democrats, David Heath demanded Mr Reid come to the house on Monday to confirm a police investigation had been launched.
Andrew Mackay (Con, Bracknell) said that the delay in dealing with Liberty's FoI request was "unacceptable" and "harming good governance".
Julie Kirkbride (Con, Bromsgrove) persisted: "Given the importance of this matter and that life could have been put at stake as the police said yesterday, surely you will press your colleagues to come forward with that leak inquiry?"
David Gauke (Con, SW Herts) hit back: "In recent years the government has undertaken leak inquiries into relatively minor matters such as details of the refurbishment of 10 Downing Street."
Stressing again the gravity of the allegations, he added: "They potentially involve an interference with a criminal investigation.
"I think if anybody has prima facie evidence in respect of the leak - and I don't just mean newspaper reports - then it's a matter for a police inquiry."