Detectives were "very alert" to the activities of the cleric - jailed on Tuesday for seven years for inciting murder and racial hatred - in 1999.

But the 47-year-old, from London, was not arrested until 2004.

Anti-terror police say evidence was sent to prosecutors "on several occasions" but no action was taken by the Crown Prosecution Service.

But BBC correspondent Rory MacLean said CPS decisions not to proceed had probably been based on the proposition that a conviction was unlikely.

"Particularly if there was evidence - or at least the police had seen things like telephone intercepts - which is not allowed in British courts, the CPS has to take a view about whether a case will actually succeed at trial," he added.