London (PTI): Releasing a comprehensive counter-terrorism strategy, UK Home secretary Jacqui Smith today warned an attack on Britain involving chemical or nuclear weapons is now "more realistic".

The strategy, released in a report titled Contest Two, said the threat to the UK came primarily from four sources: the al Qaeda leadership, terrorist groups affiliated to al Qaeda, "self-starting" terror networks or lone individuals motivated by an al Qaeda-style ideology, and terrorist groups with a similar ideology to al Qaeda.

The document said al Qaeda was "likely to fragment" under international pressure, and might not survive in its current form. But its ideology will outlive changes to its structure, possibly leading to greater threat to the UK from smaller "self-starting" organisations.

The report is the first public document to contain a detailed assessment of the underlying causes of the terrorist threat and its likely future direction. It contains a stark warning about the likelihood of an attack involving a "dirty bomb".

The report says: "Contemporary terrorist organisations aspire to use chemical, biological, radiological and even nuclear weapons.