British police detained two men today, one of whom is believed to have connections with al-Qaeda and whose arrest "could be pretty big", according to a security source.
Officers from Britain's anti-terror branch arrested a 24-year-old man in the western town of Gloucester and said they had evacuated the surrounding area as they hunted for explosives.
"It could be pretty big in the sense that if we find what we suspect we are looking for, then we have got something big," a security source said.
A police spokeswoman said the man, British-born of Asian origin, had been arrested on "suspicion of involvement in the commission, preparation or instigation of acts of terrorism".
Home Secretary David Blunkett said: "It is believed by the security and special branch services that this man has connections with the network of al-Qaeda groups.
"We would not have taken these steps if we didn't believe that this individual posed a very real threat to the life and liberty of our country," he told BBC television.
Sky News reported that the arrested man had links to Richard Reid, a British-born follower of Osama bin Laden, who was sentenced to life in prison in January for trying to blow up a transatlantic flight with explosives stuffed in his shoes. Police refused to comment.
A 39-year-old man was also arrested by anti-terror police in Manchester, Scotland Yard said. However the source said this arrest was unconnected to the operation in Gloucester.
Britain's most senior police officer Sir John Stevens said last week that London would be on high terror alert for the foreseeable future after previously warning that an attack on the UK was almost inevitable.
Earlier this month, security services were moved to a heightened state of alert after they received warnings of a possible attack by bin Laden's al Qaeda group, blamed for the September 11, 2001 attacks on the United States.