Three Asian-origin men were being questioned by British police on Friday after being arrested in connection with the 7/7 suicide bombings in London that killed 52 people.
Two men, aged 23 and 30, were arrested on Thursday at Manchester Airport when they were due to board a flight to Pakistan. A third suspect, aged 26, was held in Leeds.
Scotland Yard sources said the three men, all of Asian origin, are suspected 'backroom boys' who could have provided help and support for the four suicide bombers.
They are being held on suspicion of the commission, preparation, or instigation of acts of terrorism.
Fifty-two people died in July 7, 2005 after four suicide bombers attacked three London Underground trains and a bus. The four bombers died in the explosions, and no-one has yet been charged over the attacks.
Anti-terrorist police admitted that they were forced to seize prematurely three men because two of them were about to board a PIA flight to Islamabad. The three men are being interviewed by anti-terrorism officers at Paddington Green police station in London.
Anti-terror cooperation with Pakistan has become strained recently and surveillance of the men overseas could have been difficult. The men arrested are thought to have been under surveillance for some time and the decision by two of them to leave Britain spurred police to make their move.
Officers on Thursday searched five homes in Beeston area of Leeds where Mohammed Siddique Khan, the leader of the group, and two other bombers were based. BBC named one of the men arrested as Mohammed Shakil, a 30-year-old father-of-three from the Beeston area of Leeds.