A terrorism expert says an initial Home Office warning of a terror attack using chemical or radioactive weapons is still a "possibility".

Confusion surrounds a briefing given by the government to journalists and later withdrawn which warned Britain was at risk from "dirty" radioactive or chemical bombs at the hands of terror network al-Qaeda.

Professor Paul Wilkinson of the Centre for Terrorism Studies at St Andrews University said it was a "possibility" that the threat did exist and that the original Home Office warning was "not inaccurate".

It is wise to be prepared for a long and difficult battle to eradicate this dangerous network

Professor Paul Wilkinson

"We know the American intelligence authorities are quite convinced that al-Qaeda have been examining the possibility of using a dirty bomb in the United States and of course we have known for some time that al-Qaeda's been seriously pursuing chemical and biological weapons," he told BBC Radio 4's Today programme.

"So it's sensible to look at the worst case and also to take into account, as the earlier alert from Mr Blunkett said, that the more traditional forms of terrorism are still a danger.

"We could have for example suicide hijacking, we could have attacks on particular areas, particular buildings with truck bombs."

Prof Wilkinson says chemical attack is possible

Professor Wilkinson said he thought the reason the Home Office had withdrawn the warning was that it did not want to highlight specific threats.

"They wanted to point out that the threat was from a whole variety of tactics," he said.

It was "wise" for the Home Office to let the public know of the whole spectrum of threats, he added.

Central and local government and emergency services were preparing for the possibility of such attacks, Professor Wilkinson said.

He added everyone should make security a part of everyday life.

"It may be unwelcome advice, we would like to feel that this threat has disappeared, but al-Qaeda is still in business despite the successes that have been achieved in the war against terrorism.

"It is wise therefore to be prepared for a long and difficult battle to eradicate this dangerous network."