A senior intelligence source has revealed that the figure is a "conservative" estimate of the threat facing the country from UK-based Islamist suicide bombers.

The would-be killers are among 2,000 extremists who the security services have said are based in Britain and actively planning terrorist activity of some kind.

The figures are contained within a secret government report on the "enduring terrorist threat" facing the UK from al-Qaeda and affiliated organisations, The Sunday Telegraph has been told.