Hasib Hussain, 18, the youngest of the four suicide attackers, detonated his device on a number 30 bus nearly an hour after explosions had ripped through three Tube trains.
Evidence already heard by the inquest suggests Hussain planned to set off his bomb on the Underground at the same time as his fellow terrorists, but it apparently failed to go off and he was captured on CCTV emerging from the Tube at King's Cross station and joining the melee of commuters moved out after the initial blasts at about 8.50am.
Chilling camera footage shows Hussain rummaging in his explosives-filled rucksack outside a WH Smith store in the station and then buying a replacement 9-volt battery for the device.
The teenager inserted the new battery into his bomb in a nearby branch of McDonald's, police believe, before boarding a number 91 bus and then a number 30 bound for Hackney in east London.
Sitting at the rear of the upper deck, he blew himself up at the junction of Tavistock Square and Upper Woburn Place at 9.47am.
The blast was so loud some emergency workers deep underground rescuing victims of the King's Cross Tube attack thought it might be a secondary explosion on the train.
The loss of life in the bus bombing could have been worse if it had not taken place outside the headquarters of the British Medical Association (BMA), the inquest has heard.
Doctors attending meetings at the BMA rushed outside and began treating the injured and dying immediately.
The emergency services also reached the scene far more quickly than they did in the case of the Tube bombings, where the cramped, dark conditions hindered the rescue effort.
Most of the passengers on the number 30 had ended up on the bus after their journeys to work were disrupted by the earlier explosions on the Underground.
Many had phoned family and friends to reassure them they were safe before Hussain detonated his bomb.
The inquest will examine the question of whether Transport for London should have shut down the capital's entire transport network after the Tube bombings.