The four bombers were careful not to arouse suspicion and employed methods which seem to have been effective "in disguising what they were doing from others".

Readily available, relatively inexpensive materials were used to make the explosives.

The cost was about £8,000, mainly for overseas trips, the bomb making equipment, rent, car hire and other UK travel.

It is believed that the attacks were self-financed, with Khan – who had a £10,000 bank loan - providing most of the money.

Such funding would be "extremely difficult to identify as related to terrorism of other serious criminality", the report said.

It is thought planning of the attack started soon after Khan and Tanweer returned from Pakistan in February 2005.

A "practice run" involving Khan, Lindsay and Tanweer, but not Hussain, was carried out on 28 June.

CCTV showed them buying tickets before they boarded a train to King's Cross, where they arrived at 0855 and made their way to the Underground network.

They were seen at Baker Street at midday before they returned to King's Cross at 1250 and arrived back in Luton 50 minutes later.

Tickets found at the bomb-making factory suggest there were also visits to London in mid-March.

It is likely that the group would have needed to carry out at least one test explosion, the official report suggests. But it is not known when and where this took place.

There were also suggestions that Khan was worried he was under surveillance.

The group were careful about the use of mobile phones and used hire cars for activities connected with planning.