Campbell, aged 46, had been expected to quit later this year, but the timing of his announcement on Saturday - during a high-stakes inquiry involving whether Britain hyped the case for war - caught political observers unaware.
Campbell is credited with masterminding the slick media strategy that helped Blair's Labour Party regain power after 18 years in the wilderness.
The Cabinet Office is also expected soon to announce the outcome of a "radical" review of Government communications by an outside team led by Bob Phillis, chief executive of the Guardian Media Group.
A leading member of the review team was David Hill, who is to take Campbell's place as the Prime Minister's chief spin-doctor.
Hill is already under fire over his financial interests.
The Sunday Times has reported that Hill owns about 95,000 share options in a lobbying and public relations firm. The paper said he was unable to cash in the options for several years.
A spokesman for Downing Street said yesterday that Hill could comply fully with the rules governing conflicts of interests.
Mandelson said at the weekend: "The Phillis review has nothing to do with me. I don't want to talk to you any more. I'd like to have my Saturday back."
But a Government insider said: "The key thing people have missed is that Peter Mandelson is the brains behind this whole thing. He is the one advising Tony Blair. The King of Spin is in charge of the death of spin."
Putting a civil servant with the rank of Permanent Secretary in charge of the Government's information service will eliminate the possibility that another political adviser could have the same influence as Campbell.
In 1997, Campbell and Jonathan Powell, Blair's chief of staff, were given authority to issue instructions to civil servants - other political aides can only offer advice to politicians.
Hill, like other political advisers, will be supervised by a Permanent Secretary.
The changes have alarmed other ministers, who fear it will centralise Government communications under Blair's personal control.
The leader of the Conservative Party, Iain Duncan Smith, writing in the Independent on Sunday, said: "As Alastair Campbell goes ... it's not the resignation of the servant that matters but the departure of his master. The real Downing Street director of communications must go: Tony Blair himself."
* Meanwhile, as the Hutton Inquiry prepares to hear evidence from the widow of Government scientist David Kelly, the Observer has published an article Kelly wrote before his death.
Kelly wrote that, although the threat posed by Baghdad was "modest", only regime change could avert the long-term threat of its development of weapons of mass destruction.
The Observer said Kelly's article would be presented as evidence to the inquiry into his death, which enters its fourth week tonight (NZT).
- INDEPENDENT , AGENCIES
Hutton inquiry website
British Parliament Foreign Affairs Committee transcript:
Evidence of Dr David Kelly
Key players in the 'sexed-up dossier' affair
Herald Feature: Iraq
Iraq links and resources
