Anti-terrorist police yesterday admitted that they were forced to seize prematurely three men suspected of helping the July 7 suicide bombers because two of them were about to board a flight to Pakistan.

Detectives decided to detain the men at Manchester airport at lunch-time. Anti-terror co-operation with Pakistan has become strained recently and surveillance of the men overseas could have been difficult.

A third man was arrested in Leeds three hours later. Last night the suspects were being held in the high-security wing of Paddington Green police station, West London.

The arrests, by unarmed detectives, are the first significant move for nearly two years by officers from Scotland Yard’s Operation Theseus team, which has been investigating the attacks.

Scotland Yard sources say that the three men, all of Asian origin, are suspected “backroom boys” who could have provided help and support for the four suicide bombers before they killed 52 London Underground and bus passengers two years ago. A senior counter-terrorism source said: “We have never believed that it was possible for the four bombers to have done this on their own. Other people must have known and helped them.”

Detectives fear that a lengthy investigation could have been compromised had the suspects left Britain to be out of reach in Pakistan.

Last month on an official trip to Pakistan, Margaret Beckett, the Foreign Secretary, said that Britain was concerned about the lack of cooperation over the extradition of Rashid Rauf, who is wanted over the alleged airline bombing plot last summer.

The men arrested are thought to have been under surveillance for some time and the decision by two of them, a student aged 23 and a 30-year-old, to take flight PK702 to Islamabad in Pakistan yesterday spurred counter-terrorist officers to make their move. Police then arrested the third man, aged 26, at his home in Leeds. Under anti-terror laws they can be held for up to 28 days without charge.

Last night officers were searching five homes in the Beeston area of Leeds where Mohammad Sidique Khan, the leader of the group, and two other bombers were based for a time. One of the addresses raided by the police was the home of a taxi driver who was married with three sons. The man handed in his notice on Thursday last week, telling the owner of the taxi firm, Gee Gee Cars, that he was about to spend the next four months in Pakistan.

Other officers examined an address and a college in East London used by one of the three under arrest.

As the searches got under way in Beeston,Chief Superintendent Mark Millsom, from West Yorkshire Police, said officers were not looking for explosives and there were no plans to cordon off streets.

The July 7 bombers, who included Khan’s lieutenent Shehzad Tanweer, Hasib Hussain and Jermaine Lindsay attacked rush-hour trains on the Circle, Nothern and Piccadilly lines and a No 30 bus in Tavistock Square. In 2005 one man was arrested and released without charge and a second was convicted for wasting police time. There had been no other arrests until yesterday.

The three held yesterday will face questioning over whether they knew what was being planned and helped the bombers. Police are keen to see if anyone stored material for the bombers, lent them mobile phones, put them up or gave them lifts.

Lord Carlile of Berriew, the independent reviewer of British antiterrorist laws, told The Times last night that the 7/7 bombings looked increasingly like the work of an international group rather than a “lone pack of wolves”.

He said that the inquiries by police and security agencies into 7/7 were likely to continue for some time as they investigated connections with international violent jihadists. He added: “In my view it’s completely unsurprising that this inquiry has moved on to a much broader footing and I would expect it to continue for some years to come. “

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