David Blunkett, the Home Secretary, is due to announce on Wednesday that MI5’s budget is to rise by 50 per cent to train the young men and women. The biggest recruitment drive since the Second World War will expand the staff to about 3,000, and MI5’s budget to £300 million.

Each new recruit who passes the initial interviews will have to have his or her personal life checked before being given access to classified intelligence. Whitehall officials said that the unprecedented expansion in MI5, the domestic Security Service, was going to be a significant challenge. Vetting of every applicant’s background is a crucial part of the process to find suitable candidates for operating in a largely secret world where discretion is obligatory. It is expected to take up to three years to recruit another 1,000 people, who will be allowed to tell only their close family or partner that they have even applied for such a job.

The decision by the Government to approve such a significant increase in MI5’s budget is a reflection of the growing concern that Islamic terrorists sympathetic to the cause of Osama bin Laden’s al-Qaeda organisation are being recruited in Britain.

Part of the recruitment drive will be to increase the number of applicants from ethnic minorities. In the past 12 months, 9 per cent of successful applicants were from ethnic minorities, the highest figure for two years.

However, there are no moves at present to change the strict eligibility rules for MI5 employment: applicants are required to have at least one parent who is British-born and to have been resident in this country for a minimum of ten years. Whitehall officials said MI5 would be as flexible as possible because of the need to have more ethnic-minority recruits.

To improve operations against suspected terrorists in this country, MI5 is looking to increase the number of specialists in surveillance, intelligence analysis, communications, IT and languages, particularly Arabic and Farsi.

Britain has been on the second-highest state of alert since before Christmas, following warnings to ministers by Eliza Manningham-Buller, Director-General of MI5, that organisations such as al-Qaeda were determined to carry out attacks in this country. A number of plots targeting Britain and British interests abroad have been thwarted during the past three years.

The Government allocates more than £1 billion a year to the security and intelligence services — MI5, MI6 and GCHQ, the signals eavesdropping centre at Cheltenham. However, MI5 has traditionally been allocated the smallest budget, with the largest proportion, more than half, going to GCHQ because of its dependence on advanced and very costly computers.

Mr Blunkett’s announcement on Wednesday will be a recognition that MI5 needs more money and a bigger staff to protect this country from terrorism.

Patrick Mercer, the Conservative spokesman on homeland security, welcomed the expected rise in funds for MI5.

However, he said: “We should not underestimate the difficulties of recruiting extra officers. We wish the Government had moved faster on this issue because it will take some time for these people to become effective.”

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