At the heart of the Hutton inquiry is Blair's corruption of the civil service

The Government is reaping the whirlwind of six years during which it suborned the information offices for its own political purposes

By the end of this week we may finally learn the full extent to which the machinery of government information has been politicised since Tony Blair took office in 1997. The appearance of Alastair Campbell, later today, as one of Lord Hutton's star witnesses, has the potential to be one of the most electrifying political events of modern times.

Mr Campbell has apparently been schooled, while on holiday, by Treasury counsel in preparation for his ordeal. Hopefully, they will have been training him to present himself as a shrinking violet. And they would have done him the best favour if they have simply advised him to tell the whole truth and nothing but the truth. Any attempt to take his spin machine into Courtroom 73 at the Royal Courts of Justice could come dreadfully unstuck.

Unfortunately, Mr Campbell is not known for his humility. But, if he is not to damage further the Prime Minister's already tarnished reputation, he will need to convey the impression, if only an impression, that he is nothing but a minor functionary - just a "middle-ranking temporary civil servant".

The Government is now finally reaping the whirlwind of the past six years during which it has suborned the civil service information offices for its own political purposes. But whether it will manage to maintain its iron grip on all the witnesses in the Government's employ remains to be seen. We saw, last week, how difficult it was for the BBC to get all its witnesses to sing from the same songsheet. Individual civil servants may also be prepared to go only so far in their willingness to lay down their careers for Mr Blair.

According to the former BBC journalist, Nicholas Jones, writing in his book Control Freaks, the rot set in when New Labour politicised the Central Office of Information in 1997. The Prime Minister asked the then Permanent Secretary, Sir Robin Mountfield, at the now-defunct Office for Public Service, to carry out a review of the work done by civil service information officers "to raise their game". Writing an article for this newspaper, after his retirement, in the wake of the Jo Moore - "today is a good day to bury bad news" - affair, Sir Robin insisted that his recommendations on new working practices had reasserted the conventional neutrality of government press officers.

But even he noted that this Government had used special advisers far more intensively than its predecessors in press communication, side-stepping the official machine. "At the heart of Government, the position of special advisers is becoming more powerful and potentially dangerous, particularly in No 10, the Cabinet Office and the Treasury. The effect is not so much interference in civil servants' impartiality, as the virtual marginalisation of orthodox civil service advice".

This marginalisation reflects itself at every level of Government - even beyond the information offices - with civil servants' advice only being taken if it fits the arguments the Government has already decided to deploy. Evidence is growing that ministers are no longer interested in weighing up dispassionate advice as to the pros and cons of a particular policy. Rather, individual snippets of advice, lifted out of context, are seized upon to suit the justification of a policy already decided upon.

This is bound to have a corrupting effect on even specialist civil servants, who are under growing pressure to tailor their advice and research to fit only with what ministers and their special advisers and political press officers want to hear. If a civil servant continues to give contradictory advice, the fear of being regarded as awkward or cussed is then likely to develop into a fear of being blocked for further promotion. So the independence of the service is inevitably undermined as civil servants themselves are pressured and encouraged to compromise truth and integrity.

Some would argue that this Government has merely extended a practice which was already developing under Margaret Thatcher, and Sir John Nott, the former Tory defence minister during the Falklands crisis, certainly has harsh words for her press secretary, Bernard Ingham, in his autobiography. "Bernard Ingham was a constant nuisance throughout the Falklands campaign - jumping up and down and causing no end of difficulty, criticising the Ministry of Defence on trivial issues. But I was determined to keep the press under the tightest Ministry of Defence control and, as far as possible, away from No 10 with its obsession for background briefing and for spin."

Sir John hit on a masterstroke by choosing a Ministry of Defence official, Ian Macdonald, as the press spokesman. He spoke slowly and methodically, uttering each word "as if he was savouring a delicious plum". It was painfully obvious that he could only speak the truth. But there was criticism about the style from Mr Ingham for a "more professional style" so Sir John removed him for a weekend and put on a parade of "polished young officers" to do the job. "Sure enough there was a surge of demand for us to reinstate him."

But this episode clearly underlines the limits of Bernard Ingham's powers to influence and control other government departments. And while no one would doubt that he was equally as forceful as Mr Campbell in making his case for his boss, he was obliged by the system, which he did not dream of altering, or even challenging, to observe the proprieties of the time.

By contrast, Pam Teare, the head of news at the MoD, clearly indicated - if by implication - in her testimony yesterday, just how much she was subjected to Downing Street influence in the decision to confirm Dr Kelly to journalists if they put his name to her. The traffic between Downing Street and the MoD appears to have been non-stop, once Geoff Hoon first conveyed the name of Dr Kelly to Alastair Campbell. According to the former Northern Ireland press officer, Andy Wood, speaking on BBC's The World at One, Miss Teare would never have disclosed Dr Kelly's name to the media if she had not had approval from above - from Mr Hoon, the Permanent Secretary and, crucially, from Downing Street. It boils down, said Mr Wood, to a lack of trust between Downing Street and professionals working in other information divisions "to get it right, off their own bat".

This probably also accounts for the reason why Jonathan Powell, in his evidence yesterday, confirmed that Downing Street was so heavily involved in considering whether to submit Dr Kelly to the Foreign Affairs Select Committee. It may have been Mr Hoon who said that, for presentational reasons Dr Kelly should go before the committee. And it may have been Mr Hoon who signed the letter (drafted by Mr Campbell) to the BBC demanding that they confirm the name of Dr Kelly, but there can be little doubt that he was merely a cipher for Downing Street. No wonder he promises to be robust in defending himself against the weekend briefing that he is to be the fall guy.

mrbrown@pimlico.freeserve.co.uk