Detectives released new CCTV images of the bombers at Luton station and then at King's Cross station in London on the morning of June 28.
The images show Mohammad Sidique Khan, Shehzad Tanweer and Germaine Lindsay on an apparent reconnaissance mission to the capital.
They are dressed in trainers, casual trousers and T-shirts and two of them are carrying rucksacks.
It was also revealed today that the bombers used a peroxide-based explosive to carry out the bombings which killed 52 innocent people.
Senior anti-terrorist detectives confirmed that they had found two other viable devices in a car used by the bombers which was left parked at Luton station.
Two bottles of peroxide explosive encased in nails were found in a bag under the front passenger seat of a silver Nissan Micra which three of the bombers had used to travel to Luton on the day of the attacks.
Deputy Assistant Commissioner Peter Clarke, head of Scotland Yard's Anti-Terrorist Branch, revealed details today of the bombers' dummy run.
Sidique Khan and Tanweer met Lindsay at Luton station at around 8.10am. They bought tickets and caught a Thameslink train to King's Cross.
Further CCTV footage recovered by police shows them arriving at 8.55am. There was a further sighting of the three at Baker Street at midday.
They were captured again on CCTV leaving King's Cross at 12.50pm before arriving back in Luton at 1.40pm.
Detectives are anxious to find out what the men were doing during their three-hour trip to London, and in particular, whether they met anyone.
Mr Clarke said: "The implication is that they were possibly conducting reconnaissance on that day. We know that is part of a terrorist's methodology: to check timings, layout and security precautions.
"We are trying to reconstruct their movements as far as we can on that day. What we want to know is where else they went and did they meet anybody else while they were in London?
"If any member of the public thinks that they know something about the movement of these men on that day, they should call us on the anti-terrorist hotline."
The images of the men on June 28 were discovered among the 80,000 CCTV tapes seized by police since the investigation into the terror attacks began.
They show three of the four bombers apparently relaxed and in casual clothes mingling among ordinary commuters just 10 days before they unleashed death and carnage on the capital.
Mr Clarke revealed that detectives had also established more details about the movements of the four suicide bombers on July 7 itself.
He said that Sidique Khan, Tanweer and the youngest of the four Hasib Hussain travelled down from Leeds in a silver Nissan Micra, leaving the city at 4am.
They arrived at Luton Station at 6.51am where they met Lindsay, who had been waiting for them since 5am. The four then travelled to King's Cross Thameslink train.
Mr Clarke said both of the cars used by the bombers had been subject to a detailed forensic analysis.
He revealed that under the front passenger seat of the Micra, detectives had found a bag containing two more viable explosive devices.
These contained peroxide-based explosive and were surrounded by nails.
Detectives also found cool boxes in the rear of the vehicle, which they believe the bombers used to transport their devices that morning.
The volatility of peroxide explosives means it has to be kept cool during transportation, which is why the bombers would have needed the ice boxes, Mr Clarke said.
Forensic officers found traces of HMTP, a type of peroxide explosive, in the car and recovered 14 bits of material, some of which appeared to be component parts for other potential devices.
Mr Clarke said it would be speculation to suggest that two other bombers were supposed to turn up to join the suicide mission that day.
"We found two viable devices ready to go underneath the front passenger seat," he said.
"We do not know what these were intended for. To speculate is interesting but unhelpful, but of course it is of real concern that there were more explosives in the possession of these people on that day."
It also emerged today that detectives are searching a landfill site near Skelton Grange, in West Yorkshire, in connection with the July 7 attacks. The search area is said to be equivalent to the size of 18 Olympic swimming pools.
Detectives believe there could be a connection between the site and the apparent "bomb factory" at a flat in Alexander Grove in Leeds.
They believe that items from the so-called factory may have been put in the rubbish and found their way into the landfill site.
Mr Clarke said the search of the Alexander Grove property had taken six weeks and involved the removal of 2,000 exhibits.
He said it was too early to say how much explosive material had been in the flat but added: "This was clearly a carefully thought out and effective manufacturing facility."
A huge amount of forensic work and analysis of material was still ongoing, he added.Reuse content