Possible links between militant groups based in Pakistan and Shehzad Tanweer, a Briton of ethnic-Pakistani origin who was one of four bombers in the July 7 attack on London that killed at least 54 people.

"The Pakistan security agencies have picked up four people ... but we were not involved in this operation," a Faisalabad police official told Reuters on condition of anonymity.

One of the groups being checked is Jaish-e-Mohammad (Army of Mohammad), linked to al Qaeda and banned by Pakistan in 2002. The other group is Lashkar-e-Taiba, which like Jaish has a record of fighting in Indian Kashmir, but unlike Jaish has a reputation for tight discipline and is not known to have any operational ties with al Qaeda.

One Pakistani intelligence source said Tanweer visited Pakistan in 2003 and 2004.