Gordon Brown waded into the Damian Green affair today, saying: "No MP is above the law".
The outburst came as the Metropolitan police launched a review into their own investigation concerning the shadow immigration minister.
Mr Brown said: "MPs have to be allowed to get on with their job but no MP is above the law.
"I'm concentrating on the work in hand."
But the Met is clearly reacting seriously to the controversy around last week's arrest.
In an unprecedented move highlighting the constitutional crisis provoked by the arrest, acting commissioner Sir Paul Stephenson said: "I am properly concerned about the issues being raised within the continuing debate surrounding the ongoing investigation into the leaking of government information."
Sir Paul has appointed Ian Johnston, chairman of Acpo's crime committee and chief constable of the British Transport police, to review the police's "decisions, actions and handling of the investigation to date".
The home secretary has welcomed the review.
"In view of the gravity and sensitivity of this ongoing investigation, I spoke to Sir Paul yesterday to seek his assurance that the investigation was being pursued diligently, sensitively and in a proportionate manner," Jacqui Smith said.
"Sir Paul informed me of his intention to conduct a review of the handling of the case, which I welcomed."
The home secretary's position is becoming increasingly fragile as the story creates headlines several days after the original arrest.
Today, bookmakers William Hill gave her odds of 6/4 that she will leave her post before the next general election.
"This affair has done little for the reputations of Ms Smith and Sir Paul Stephenson, and both could yet find themselves paying a hefty price for what happened," said William Hill spokesman Graham Sharpe.
An interim report will be produced in a week and a final report in a fortnight.
The aftershocks of last week's arrest showed no signs of dissipating today, with politicians and journalists still obsessing over the issue and warnings of a boisterous debate after the Queen's Speech tomorrow.
Liberal Democrat leader Nick Clegg is understood to be meeting with his Conservative counterpart David Cameron today to discuss tactics.
Speaker Michael Martin will give a statement to MPs tomorrow at 14:30 GMT.
Last night, an email organising a meeting between ministers and the Speaker - probably to establish key facts before he has to give his version of events on Wednesday - was sent to Conservative sources by mistake.
The original recipient of the email was in the Cabinet Office, but he shares a surname with an employee in the office of Philip Hammond, shadow chief secretary to the Treasury, who ended up receiving the message.
The email revealed details of the meeting between leader of the Commons Harriet Harman and other ministers, civil servants, Commons officials and the Speaker.
Justice secretary Jack Straw, Cabinet secretary Gus O'Donnell and serjeant at arms Jill Pay are also attending.
But Tories are describing the meeting as a 'stitch-up' and have demanded an invitation.
That meeting took place at 13:00 GMT today. Afterwards, Ms Harman's office released the following statement: "The Leader is aware that the Speaker has confirmed that he will make a statement to the House tomorrow at 2.30pm.
"The contents of that statement are entirely a matter for the Speaker. There have been discussions today about issues relating to Government business."
Shadow home secretary Dominic Grieve said: "This is a completely improper meeting convened by the government privately with the Commons authorities and senior civil servants to manage the Speaker's statement without representation from other parliamentarians.
"This is precisely the sort of leak that should make it into the public domain."
The Speaker is facing considerable pressure over the affair, the most sustained media attention since his expenses were brought in to question in February.
Mr Martin's authorisation was required for police to enter parliament to search Mr Green's office, and some MPs are now asking whether he should have rejected the request.
The police investigation is understood to want to question former shadow home secretary David Davis, who was Mr Green's boss at the time of the leaks.
But sources indicate the investigating officers are nervous about dragging more politicians into the affair at a time when their own actions are under the spotlight as much as those they are investigating.
A source told the Times: "If we're going to follow the evidence we need to speak to Mr Davis. But if we do that we raise the temperature even further."
Of the police review into the investigation, Mr Davis said himself: "This is the first indication by Scotland Yard of what a serious error they've made."
The appointment of the next commissioner may also depend on the outcome of the review.
Should it be critical of Sir Paul he may have to withdraw from the race for the job, considering he was the man who sanctioned the arrest.
Yesterday Sir Paul was facing calls for him to resign as acting-commissioner, despite having only been in the job for hours.