He revealed that there were 700 casualties, 350 people were taken to hospital, 22 were still in a critical condition and one person died in hospital.
He agreed that the attack 'bore all the hallmarks' of an al Qaida operation.
Anti-terrorist branch head Andy Hayman said each of the bombs contained less than 10lbs of high explosives and they were probably placed on the floor of the three Tube trains or, in the case of the bus, on the floor or a seat.
Sir Ian said it was open to question whether the terror cell responsible for the attack was still in the UK. But he added: 'We must remain vigilant.'
Home Secretary Charles Clarke said all efforts were being concentrated on catching the bombers to stop them striking again.
He said: 'The number one purpose today is to identify the perpetrators and arrest them because there is obviously a danger if there is a group that has committed these attacks not brought to justice and therefore able to continue thinking about carrying out further attacks.'
He agreed that the authorities had 'absolutely no idea' yesterday's attacks were being planned.
But he denied claims that London's security had been compromised because of Metropolitan Police officers being deployed to the G8 summit in Scotland.
The National Co-ordination Centre confirmed that the bodies of all of the dead are to be moved to London mortuaries by noon.
Scotland Yard said there would be an increased visible police presence across the capital today. Extra patrols will include officers from the Met, City of London Police and British Transport Police.
Scotland Yard has issued a casualty hotline number on 0870 1566 344.