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Foreign Secretary William Hague's special adviser Christopher Myers, 25, has resigned after "untrue and malicious" allegations about their personal lives - but who are special advisers, and what do they do?

Clare Short disparagingly dubbed them the "people who live in the dark".

They can often be spotted darting through the television studios of Westminster with their minister, briefing papers under arm and Blackberry in hand.

Young, sharp and driven, they are often politicians-in-waiting - among former "spads" are Prime Minister David Cameron, Chancellor George Osborne and Labour's Miliband brothers, David and Ed.

Employed as temporary civil servants, they do not have to be politically impartial like their civil service colleagues.

They link together the minister, the party and the department. They are also the bridge between the neutral civil service and the politicians.

They help write speeches, some are policy wonks, while others focus on the media.

If a journalist wants to know what a cabinet minister thinks or understand what a policy is about, a call to the special adviser is one of the first ones to make.

But they are sometimes sneered at by some journalists.

Michael Jacobs, a former special adviser to Gordon Brown, told the ³ÉÈË¿ìÊÖ that the perception of the job as "illegitimate" was unfair: "No other country in the world regards advisers in this way.

"Most ministers have more than they do in the UK and I think it's important that their role is brought out because they do a rather important job."

He said while ministers needed civil servants for impartial advice, they needed special advisers to help them to make political judgements and consider different options: "They are a sort of lubricant in the machine."

Special advisers first became a permanent fixture in Whitehall in the 1970s. But their number ballooned under Labour.

In 1996 there were 38 working in government, costing the taxpayer £1.8m. In 2004 the number peaked at 84 and last year there were 74, at a cost of £5.9m.

But their expanded ranks prompted concern about their role. Critics felt a more American, politically driven civil service was sneaking in via the special advisers and a line of accountability was being blurred.

And the e-mail sent by special adviser Jo Moore after the terror attacks in the US on 11 September 2001 saying it would be a good time "to bury" bad news triggered a number of reviews into their role and power.

Another "spad"-related scandal - the discovery that Damian McBride was smearing senior Conservatives in e-mails - prompted Gordon Brown to ask the cabinet secretary to review the rules governing their behaviour.

There were suggestions that the Conservatives would drastically reduce the number of special advisers - a democracy task force headed by Ken Clarke in 2007 recommended they be halved.

But while the coalition agreement agreed to "put a limit" on their number, it did not spell out what that limit would be.

In the first list published after the May election, the number of special advisers was cut from 78 to 68.

But there have been some subsequent appointments and the list is only updated quarterly.

Among recent additions was Mr Hague's third special adviser Christopher Myers - who was reportedly paid £30,000 a year. That would put him in the lowest salary band - senior special advisers can earn between £66,512 and £142,668.

Cabinet ministers may appoint up to two "spads" each, but if they have "additional responsibility" - Foreign Secretary Mr Hague is also first secretary of state - they can appoint more, subject to the PM's approval. David Cameron has 18, deputy PM Nick Clegg has four.

So under the coalition, special advisers continue to roam the corridors of Whitehall.

Their close relationships to cabinet ministers and lobby correspondents give them influence - an influence that can hatch into a political career later on.

A successful stint as a "spad" can be a crucial political apprenticeship - as many of the current crop of professional politicians can testify - so long as they stay in the dark.