So what should be done about Damian "McPoison" McBride, the disgraced former aide at the centre of the Number 10 "smeargate" scandal? Mr McB was a spad, or special adviser, and perhaps the root of the problem is that ministers can appoint anyone they like as spads - any unelected crony will do, no matter how incompetent. What's more, there is no attempt to distinguish between spads who give policy advice, albeit from a party political perspective, and those who simply specialise in spinning and smearing.

Andrew Blick, an academic and author of People Who Live in the Dark , a history of special advisers, says it all started in 1964. After long years in opposition, the Labour government decided it needed socialist advisers to act as a counterweight to the civil service, which was suspected - unfairly - of cosying up to the Tories. The early spads were policy people, although, as Mr Blick says, malicious gossip was as popular then as now. One adviser, John Harris, later Lord Harris of Greenwich, specialised in briefing against Harold Wilson, then prime minister, and in favour of his rival Roy Jenkins. Numbers started small, but by last year there were 73 spads, with 24 at Number 10, at a total cost of £5.9m (last year 28 earned more than MPs, with salaries between £64,056 and £102,918). Former spads among senior politicians include Jack Straw, Vince Cable and, of course, David Cameron.