When that arch media manipulator Peter Mandelson pointed an accusing finger at unnamed officers in the Metropolitan Police Authority and blamed them for being responsible for a deluge of embarrassing leaks during the cash-for-honours investigation, he could hardly have paid a finer back-handed compliment to himself.

Here was an infamous former spin doctor, who was prolific in his own exploitation of leaked information, having the gall to castigate anyone else who had dared turn the tables and tried to undermine the credibility of Tony Blair and his closest colleagues.

Mandelson, like his fellow trader in confidential data, Alastair Campbell remains in denial about the damage his manipulative techniques inflicted on both the political process and the conduct of government; together they helped change the culture of Whitehall and Westminster and usher in an era where leaking has become a way of life within the state.

Instead of taking his cue from Blair and quietly welcoming the decision of the Crown Prosecution Service (20.7.2007) not to proceed with charges after Scotland Yard’s sixteen-month investigation into the sale of peerages, Mandelson went on the offensive and accused the Police of adopting the very same tactics which he himself had turned into an art form.

Mandelson’s public criticism of leaks from the Metropolitan Police (Guardian 21.7.2007) reeked of hypocrisy:

"Those who undertook this investigation used the media to create a false impression of the then Prime Minister, and to undermine public trust in the government. I feel sorry for those individuals who were targeted, but I feel even sadder for the lasting damage that has been done to British politics".

I had a ring side during the 1980s and 1990s when Mandelson and Campbell demonstrated how they were ready to leak, leak and leak again in order to promote Neil Kinnock and then Tony Blair. Their list of targets had no limits: Margaret Thatcher and then John Major, the left of the Labour Party, trade union leaders, and assorted troublesome individuals (myself included) who found their reputations trashed in the media after judicious leaks to favoured journalists.

On becoming Kinnock’s director of communications in 1985, Mandelson started off in a small way by providing regular leaks to the Sunday Times; many was the time on weekend duty when, after much endeavour, I finally obtained confirmation from the Labour Party of the stories which Mandelson had "trailed" anonymously in the Sunday newspapers.

Likewise Campbell’s first act on entering Downing Street’s as Blair’s official spokesman was to change the rules for Whitehall information officers to allow them to "trail" government decisions before they were announced to Parliament.

By the time of his own ignominious resignation in the summer of 2003, leaking had become so institutionalised within government departments that under the instruction of Labour-supporting political advisers, civil service information officers were being authorised to leak even sensitive financial information.

Campbell’s diaries The Blair Years provided countless examples of his leaks to newspapers such as the Sun and Daily Mirror. Lance Price, an eager pupil of Mandelson and Campbell, peppered his book, The Spin Doctor’s Diary, with descriptions of how during his time in the Prime Minister’s press office, Downing Street became a clearing house for leaking government secrets.

Only Mandelson could fail to see the irony of his attack on the Metropolitan Police. In the wake of the collapse of Scotland Yard’s investigation, Assistant Commissioner John Yates issued a firm denial of responsibility for the constant flow leaks.

Nonetheless as the inquiry dragged on for month after month, it became obvious that highly sensitive information was seeping out from police officers, lawyers and others in the know; even the decision of the Crown Prosecution Service not to proceed with charges was leaked to the news media the night before the official announcement.

A trawl through the newspapers which reported the ending without charges of the cash-for-peerages saga, offered numerous other illustrations of the way leaking has become an established way of targeting opponents and putting pressure on the government.

For example, under the headline "We have run out of troops, says head of Army", the Daily Telegraph revealed exclusively the contents of a leaked memorandum written by General Sir Richard Dannatt; it warned that Army reinforcements for emergency operations were "now almost non-existent" because of the strains imposed by Iraq and Afghanistan.

Any journalist reading the document would have reached the obvious conclusion and suspected this was a document which had been written for leaking; all the tell tale signs were there and Dannatt was no stranger to the headlines having provoked a political storm in October 2006 when he told the Daily Mail that the continued presence of British troops "exacerbates the security problems" in Iraq.

Mandelson may well live to regret joining in those who attacked the conduct of Scotland Yard. Once the House of Commons Public Administration Select Committee resumes its own stalled inquiry into the allegations over the sale of peerages, Assistant Commissioner Yates could well be asked to let Mps have access to the contents of the report which the Police submitted to the Crown Prosecution Service.

While the CPS concluded there was "insufficient evidence to support proceedings against any individual", there is likely to be a considerable quantity of potentially incriminating documentation in the form of Downing Street emails and correspondence and if this information is placed in the public domain it will help resolve many unanswered questions.

After the row between the government and the BBC over weapons of mass destruction in Iraq and the death of the weapons inspector Dr David Kelly, Lord Hutton exonerated Blair, Campbell et al. But the Hutton Inquiry broke new ground by publishing evidence which would not otherwise have seen the light of day. So it is with the cash-for-peerages investigation.

Similarly the CPS has broken with convention by releasing details of the legal argument surrounding its decision not to mount charges. Who knows what the reaction might be if the public get sight of the evidence which Assistant Commissioner Yates appeared to give every indication of believing did warrant a prosecution.

Nicholas Jones is the author of Trading Information: Leaks, Lies and Tip-offs , published by Politico’s, July 2006.