And Prime Minister Gordon Brown indicated that there may be further inquiries after the conclusion of the investigation, which last week saw the arrest of Conservative immigration spokesman Damian Green on suspicion of conspiring to commit misconduct in a public office.
The Met's Acting Commissioner Sir Paul Stephenson called in the head of the British Transport Police Ian Johnston to look into his force's "decisions, actions and handling of the investigation", Scotland Yard revealed today.
The decision reflects Scotland Yard's concern about the mounting political furore over Mr Green's arrest and the search of his office in the House of Commons. The Ashford MP, who denies all wrongdoing, has been bailed until February.
Sir Paul said he was "properly concerned about the issues being raised within the continuing debate surrounding the ongoing investigation".
Mr Johnston, who chairs the Association of Chief Police Officers crime committee, will deliver his interim findings to Sir Paul within seven days, followed by a full report in two weeks. In the meantime, the investigating team will hold discussions with the Crown Prosecution Service on the next steps in the investigation.
Mr Brown today hinted that this may not be the last inquiry into the affair, telling reporters: "There is going to be a time when all these things are going to be investigated and reviewed after the police have finished their work and MPs, of course, have got to be allowed to get on with their job.
"But no MP is above the law. There has got to be operational independence for the police. The police have got to be able to get on with their job without interference by politicians."
Home Secretary Jacqui Smith welcomed Mr Johnston's review, and restated her support for the police's operational independence.
"No-one should seek to prejudice a police investigation in any way," she said. "These are very serious matters, and the police should be free to pursue their investigations without fear or favour."
But Tories seized upon her revelation that she sought and received assurances from Sir Paul yesterday that the investigation was being carried out "diligently, sensitively and in a proportionate manner".
Shadow home secretary Dominic Grieve said Ms Smith appeared to be "finally... (doing) her job as Home Secretary in scrutinising the way the police are doing their job - not telling them what to do, but asking the searching questions that are her responsibility.
"This should have happened before any action was taken against Damian Green and the failure of that to happen has never been properly explained."
Mr Grieve called for an urgent statement from Ms Smith about the instigation and progress of the Home Office inquiry into a series of leaks over the past year, and how it developed into a police investigation.
Ms Smith was today meeting representatives of the Commons authorities and Leader of the House Harriet Harman to discuss arrangements for tomorrow's statement by Speaker Michael Martin about the police search of Mr Green's office in the Palace of Westminster.
The raid has sparked widespread anger, with some MPs condemning it as a breach of parliamentary privilege.
Mr Martin is expected to make his statement tomorrow afternoon, after MPs return from hearing the Queen's Speech in the House of Lords.
Senior Tories including former Home Secretary Kenneth Clarke and ex-shadow home secretary David Davis are expected to lead demands for a full debate, which may delay the scheduled debate on the Government's legislative programme for the coming year.
Some Tory MPs are understood to be considering tabling a motion of no confidence in the Speaker, which could potentially pave the way for his removal. Harwich MP Douglas Carswell said he was not alone in thinking Mr Martin was "not up to the job".
"It seems that the one person in Westminster who is still hiding behind the fiction that correct process was followed is Michael Martin," Mr Carswell told BBC News 24.
"It's simply not acceptable that the Speaker of the House of Commons should not understand the enormity of what he's allowed."
Liberal Democrats are expected to push for a full examination of events.
Leader Nick Clegg warned: "If we let the Damian Green incident just pass, I think we'll go down a slippery slope of letting the Government do whatever it likes between one election and the next, and that's not going to help anyone in the country as a whole."
London Mayor Boris Johnson, who chairs the Metropolitan Police Authority, welcomed Mr Johnston's appointment: "It is right that someone independent should look closely at how the police conducted this investigation and whether the actions and decisions they took were proportionate and necessary and I am pleased at the common-sense approach this represents."
MPs on the House of Commons Home Affairs Committee are to question ministers over the handling of the leak inquiry when they appear to give evidence over the coming weeks, said committee chairman Keith Vaz.
The senior Labour backbencher said there were "clear concerns and questions" over the conduct of the investigation so far, and welcomed the decision to call in Mr Johnston.
Meanwhile, the Association of Police Authorities urged ministers to rethink plans to introduce direct elections to the bodies which oversee local policing, in order to avoid future political rows over investigations.
Only in London are the police accountable to a single political party, through the Home Secretary or Mayor, while in other parts of England and Wales police authorities are appointed in a way which prevents any party holding a majority, said APA chairman Bob Jones.
He added: "Outside of London, allegations of Chief Constables working to a party political agenda are rare, as the balanced structure of police authorities which oversee local policing means this allegation has no credibility.
"The proposals from all three front benches, including the Government's proposals to introduce direct elections to police authorities, would re-introduce a party political majority to whole swathes of the country, opening up even more allegations of a party political agenda to the detriment of both policing and politics."
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