Ministers have faced intense criticism over the use of stop-and-search powers against political protesters, including opponents of ID cards and a heckler at last year's Labour conference.

Now it appears people found in possession of anoraks and notepads in the vicinity of railway stations could be subjected to laws aimed at al-Qaeda.

In recent weeks, the UK's large railway enthusiast community has been gripped with tales of persecution by security forces. Spotters at Motherwell, Reading, Slough and Croydon stations are said to have been detained under the Terrorism Act.

As a result of those incidents, Mr Clarke has faced questions in parliament.

"While police do not target the use of anti-terrorist powers against rail enthusiasts, photographing trains and railway stations can be an indicator of possible hostile terrorist reconnaissance," Mr Clarke said.

He added: "Officers who encounter such activity may deem it appropriate to use stop-and-search powers."

Many spotters urge the British police to take a lead from the US, where spotters have semi-official recognition as a potential asset in the war on terrorism.