David Cameron's announcement of a major expansion of Britain's intelligence workforce is set to trigger a frantic recruitment drive at spy agencies.

But the announcement poses the question: where will MI5, MI6 and GCHQ find 1,900 new operatives and how quickly can they do it?

Increasing the 12,700-strong staff by 15% is no small task, and is likely to see a continuation in the trend for recruiting from all strands of society.

Earlier this year MI5 launched an advertising campaign to hire intelligence officers who can convince some of the thousands of jihadists it is monitoring to become spies.

The campaign was aimed at operational intelligence officers who could be deployed to "turn" members of Islamist networks or hacking groups so security services could obtain crucial information.

MI5's jobs website currently states that it is open to applications from school leavers, graduates and "experienced hires".

There are currently 16 vacancies listed. Roles include intelligence and data analysts earning a starting salary of £28,821, with successful candidates providing intelligence officers with "essential information that directs their work".

A "physical security adviser" and an "explosive chemist physical security specialist" are also being sought, with pay packages of up to £42,764 and £53,284 respectively.

MI5 is listed as one of the top UK employers for lesbian, gay, bisexual and transgender people, according to the Stonewall Workplace Equality Index.

It says that 40% of its staff are women and more than half are under the age of 40, with an increasing number - currently around 8% - from black or ethnic minority backgrounds.

In order to apply for roles at the service, applicants must be British citizens - either born in the country or naturalised - and at least 18 years old.

An overnight influx of new agents seems unlikely, not least because the recruitment process is necessarily rigorous.

All candidates undergo comprehensive vetting before they are able to obtain the highest form of security clearance. This process can take as long as nine months.

GCHQ could turn to the country's burgeoning number of IT whizzes as it attempts to swell its ranks.

In May the Government's listening post began openly recruiting for computer network operations specialists to help fight terrorists and cyber criminals for the first time.

And last month it emerged that MI6 was advertising for intelligence officers on the parenting website Mumsnet.