July 7 Tube bomber argued with cashier shortly before blast

Another of the terrorists - the teenager who destroyed a double-decker bus - was also captured on surveillance cameras wandering around the streets of London, "bumping into people", before detonating his rucksack bomb.

New details of the behaviour and last movements of the four suicide bombers, who killed 52 people, were disclosed by a representative of the Metropolitan Police Anti-Terrorist Branch, the magazine Police Review has reported.

The counter terrorist expert also told a seminar that the policing bill for the attacks on 7 July and the failed bombings on 21 July so far stands at £77m.

He warned traffic officers that the four terrorists - Mohammad Sidique Khan, 30, Shehzad Tanweer, 22, Germaine Lindsay, 19, and Hasib Hussain, 18, - did not fit the preconceived terrorist profile.

Tanweer hired a Nissan Micra and is believed to have been used to bring the other two Leeds-based terrorists, Hussain and Khan, to Luton railway station, from where they took the train into London for the bombing mission.

As an example the unnamed official told delegates that Tanweer argued with a cashier that he had been short changed, after stopping off at a petrol station on his way to the intended target in London.

The official told the seminar held in Preston, Lancashire two weeks ago: "This is not the behaviour of a terrorist - you'd think this is normal.

"Tanweer also played a game of cricket the night before he travelled down to London - now are these the actions of someone who is going to blow themselves up the next day?

"I've seen the CCTV footage of these people. They do not appear to be on their way to commit any crime at all. The Russell Square bomber [Hasib Hussain] is actually seen going into shops and bumping into people [prior to his attack].

"We have been told in the past that the normal age [for a terrorist] is about 30 ... that profile is totally wrong."

Fresh details about the apparent confusion and disorientation of the youngest bomber, Hussain, follows the disclosure that he left the Underground system and wandered around the King's Cross area - at one point he was filmed going into a McDonald's take-away - before setting off his bomb on a No 30 bus in Tavistock Square, killing 13, more than an hour after the other terrorists had detonated their devices on the Tube trains.

Tanweer detonated a bomb on a Circle line train between Aldgate and Liverpool Street stations which killed seven people, including himself.

Detectives also discovered that three of the bombers - not including Hussain - had visited London and staged a practice run nine days before the attack.

The representative from the anti-terrorist branch warned officers at the seminar that terrorists may not necessarily act like people who are about to blow themselves up.

He told delegates to watch out for signs of hostile reconnaissance. He added: "They will be looking to obtain a profile of the building, determine the best mode of attack, and determine the optimum time to conduct an operation."

The official asked officers to look out for groups of two or more people taking significant interest in the location of CCTV cameras, and also vehicles parked outside a building with people staying inside the vehicle longer than usual.