"Special adviser" is a term that usually conjures up the image of a Machiavellian figure emerging from the shadows to put a favourable spin on a government announcement. Indeed the dust jacket of Andrew Blick's book depicts Alastair Campbell (looking suitably homoerotic) and Jo Moore (looking suitably faux contrite) and sports a title derived from Clare Short's condemnation of new Labour spin-doctors.

But look within the hard covers of this fascinating and carefully researched book - of interest to politics students and the public alike - and a very different story emerges. Blick locates the need for special advisers in the defects of the 1854 Northcote-Trevelyan report, which led to a Civil Service staffed by Oxbridge-educated generalists. The emphasis on abstract intelligence (as opposed to technical knowledge or commercial experience) worked at the time of the laissez-faire "night watchman" state. But three things altered the political environment: national economic decline, the growth of "new liberal" interventionism and the enfranchisement of the working class. Public administration by brilliant Oxbridge-educated classicists no longer fitted the spirit of the age.

As a result, reforming (and wartime) governments have felt the need to second "expert" advisers, whereas in times of peace or conservative dominance Sir Humphrey calls the shots. Lloyd George's "garden suburb" of special advisers was demolished after the Great War, but Churchill reintroduced special advisers - including John Maynard Keynes, Harold Wilson and Lord Cherwell - during the Second World War.

However, the modern era of special advisers dates from Wilson's appointment of economists Thomas Kaldor and Nicholas Balogh in 1964 to help implement his modernising agenda. The 1968 Fulton report welcomed the secondment of expert advisers as an aid to efficient government.

³ÉÈËVRÊÓƵ

Conservatives tend to view special advisers as a dubious innovation, but Margaret Thatcher soon found she could not live without them as she was herself something of an anti-establishment outsider. But, with the notable exception of Alan Waters, the advisers of the Thatcher era tended to be businessmen, such as John Hoskins, rather than economists.

John Major's tally of special advisers (38) was doubled within six months by the Blair Government, and the distinction between "expert" and "political" advisers elided. Although the media has focused on the special adviser as spin-doctor, advisers have contributed enormously to policy development. Michael Barber was the intellectual source of Labour's education policies, whereas David Miliband was the architect of the Third Way and the author of the 1997 manifesto. When Michael Heseltine described neo-classical endogenous growth theory as "Balls not Brown", he might also have included the entirety of new Labour economic policy, from the five euro criteria to the transfer of monetary policy to the Bank of England.

³ÉÈËVRÊÓƵ

One might well ask what the implication of all this is for parliamentary democracy. When there was a disagreement between Andrew Adonis and Estelle Morris over university tuition fees, it was the minister who resigned. Most of the growth in special adviser numbers has been in the (de facto) Prime Minister's department, and Blick appears to share the view of MP Graham Allen (who happens to be his boss) that we have acquired a British presidency by stealth. The difference between Jonathan Powell (Tony Blair's chief of staff) and his US namesake, Colin Powell, is that the latter has to work in the glare of media scrutiny, whereas the former successfully refused to appear before the Public Administration Committee.

Perhaps Allen is right and we need the checks and balances of the US Constitution, but I am more inclined to think that the chasm between Blair's special adviser cronies and the huddled masses on the parliamentary back benches calls into question the whole party system.

Keith Sutherland is the author of The Party's Over .