The culture of spin, rebuttal and news management, once so feared by Labour's political opponents, is now at the centre of the most serious crisis to engulf Mr Blair's premiership.

Six years after Labour's hugely professional Millbank communications machine helped sweep the Government to power, the death of Government weapons expert Dr David Kelly has given a tragic edge to the debate about news management which has accompanied Mr Blair's years in power.

New Labour's reputation for exploiting the dark arts of political spin date back to the party's long years in opposition.

As practised by Peter Mandelson and Alastair Campbell, it was an aggressive mix of attacks against John Major's Conservative Government and ruthless rebuttal of criticisms of Labour.

The machine ran rings round the Conservatives' tired Government machine. But the decision to cultivate the hitherto hostile right-wing tabloid press was matched by a willingness to bulldoze news editors into promoting the party's agenda. Famously, Alastair Campbell once contacted the BBC and ITN to remind them that a forthcoming Blair speech was more important than the result of the OJ Simpson murder trial.

The practice of spin that was honed in opposition was extended into Government, with Labour apparatchiks continuing selective briefings and announcing and reannouncing policy initiatives.

The tipping point came on 9 October, 2001 when The Independent revealed that Jo Moore, spin doctor to Stephen Byers, then the Secretary of State for Transport, had dispatched an email on 11 September declaring that "it's now a very good day to get out anything we want to bury." The ensuing furore not only brought down Ms Moore and Mr Byers but also inflicted deep wounds on the Government's reputation.

But it is the war in Iraq which has tested the philosophy of political spin to the limit, with doubts about the legal and political basis for the war.

The September and February dossiers on Iraq's weapons, and the Foreign Office's document outlining Saddam Hussein's human rights abuses were based on the classic tactics of spin, designed to promote the impression of an imminent threat from Iraq.

Doubts were only heightened when Alastair Campbell appeared before the Foreign Affairs Select Committee last month to issue his personal rebuttal of Andrew Gilligan's claims on the BBC that Number 10 "sexed up" the September dossier.

The decision to release the name of Dr David Kelly as Mr Gilligan's possible source could yet prove move damaging for Mr Blair's government.

Mr Blair himself has frequently been reported as declaring the end of spin for New Labour. But observers have interpreted this as yet more spin.

Yesterday, journalists in Westminster attended a lobby briefing, the week's questions dealt with in little more than eight minutes. As Tom Kelly, the Prime Minister's Official Spokesman, left the room, he muttered to a colleague: "mission accomplished."