Three were in Pakistan together but the Islamabad government continues to protest that there is no proof of these men linking up with terror suspects inside their borders.

British investigators disagree. They claim that the British bombers met a number of known militants belonging to groups operating inside Pakistan which have close ties to the leadership of al-Qaeda.

These outlawed groups have trained many young Britons in using weapons and explosives and sent their more promising recruits to al-Qaeda terror camps. They have also groomed up to half a dozen other Britons as suicide bombers.

The links between who first “talent spotted” the four 7/7 bombers and those who persuaded them to take part in a violent attack on the country of their birth are tortuous. But investigators say the links keep leading them back to Pakistan.

Scotland Yard detectives point to the likes of the London college dropout, Zeeshan Siddiqui, who left his parents a scribbled note in 1999 saying he was leaving home to become a “holy warrior” and is now in Islamabad’s top security jail.

Siddiqui was arrested in Pakistan shortly before the first London strike for his alleged links to some of al-Qaeda’s most notorious figures.

What interests Scotland Yard’s counter-terror team are reports of Siddiqui’s rendezvous earlier this year with the one of the Leeds bombers, Shehzad Tanweer. Detectives desperately want to know what the pair discussed and who else Tanweer met during his three-month stay in Pakistan.

Tanweer is believed to have contacted a number of figures involved with the banned Jaish-e-Mohammad group, including Osama Nazir, who is facing trial for an attack on a Christian church in which two Americans died.

Jaish is known to operate in Britain, gathering support from colleges and radical mosques.

Among their recruits is the former public schoolboy and LSE dropout, Ahmed Omar Sheikh, who is on death row in Islamabad for the kidnap and murder of the Wall Street Journal reporter Daniel Pearl.

They also recruited Britain’s first suicide bomber, Mohammad Bilal, who on Christmas Day 2000 rammed a vehicle packed with explosive into an Indian military post in Kashmir. Jaish and another outlawed group, Lashkar-e-Taiba, collect more than £5 million a year in donations from Britons who believe their funds are going to charities in Kashmir.

Nobody can say for sure how many young men from Britain they have sent to terror training camps, but three of the four July 7 bombers are believed to be among them.

It is the international links of the 7/7 bombers that so disturb US investigators. Michael Sheehan, New York’s deputy commissioner for counter-terrorism, said the organisations that groomed the London bombers were active in his city.

New York’s police chief, Raymond Kelly, revealed that the bombmaker had used household items such as hair bleach and food preservatives to cook up the explosive devices. “These terrorists went to a hardware store or a beauty supply store,” Mr Kelly said.

Investigators at first thought the terrorits had used a military high explosive. Now they have discovered it was a homemade peroxide-based mix called HMDT (hexamethylene triperoxide diamine). The recipe may be available on the internet, but the degree of sophistication used in these devices suggests a practised bombmaker was used. He knew enough to buy expensive fridges for the scruffy flat in Leeds to keep his materials cool.

The bombmaker also ensured that the explosives were packed into coolboxes for the car journey south so the materials would not degrade or blow up prematurely. He used the alarm on mobile telephones to trigger the bombs, the same tactic used by terrorists in Madrid who left their rucksacks on four commuter trains in March 2004.

Pakistani ministers say that none of the 600 people rounded up in their crackdown on militants since 7/7 has anything to do with Britain’s inquiry. Yet a senior security source has told The Times that there are seven men detained who are being questioned about their associations with the 7/7 bombers.

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