The Metropolitan Police Commissioner said that London had been "very, very lucky" to escape the failed 21 July bombings, adding that there was no reason to suspect that further attacks would be confined to the transport system.
Sir Ian's comments came a week after that attempt and three weeks after the 7 July suicide bombers killed 52 people.
British Transport Police said yesterday that it had deployed an unprecedented number of officers to deter further attacks and to reassure the public.
Armed police were also on duty outside Tube stations and at many public buildings and key locations around London.
Meanwhile, anti-terrorist squad detectives continued to question Yasin Hassan Omar, the suspected would-be Warren St bomber, following his arrest in Birmingham on Wednesday.
The massive manhunt for the other three suspects continued with police searches across London.
In Stockwell, south London, a flat said to be occupied by one of the three remaining 21 July suspects was raided on Wednesday night. Nine people were also arrested during two raids in Tooting, south London.
Speaking at the first meeting of the Metropolitan Police Authority since the 7 July attacks, Sir Ian said he was confident the bombers would be caught. But he stressed that the failure of the 21 July attacks did not signify a weakening of their capability or resolve.
He said: "This is not the B team, these were not the amateurs; they only made one mistake, and we're very, very lucky. The carnage that would have occurred, had those bombs gone off, would have at least been equivalent to those on 7 July."
Sir Ian also warned the counter-terrorism effort had to remain "broad ranging" and not focused just on the Tube or the buses. "There is no reason why they are going to stay attacking the transport system," he said.
Sir Ian described the investigation as the "greatest operational challenge that the Metropolitan Police Service has faced since the Second World War". The force had received more than 5,000 calls to the anti-terrorist hotline and was examining 15,000 CCTV tapes. The entire investigation was costing around £500,000 a day.
He admitted the Met was "stretched" and there were some "very tired faces" around Scotland Yard.
The Yard last night refused to confirm reports that the man who lived at the property raided in Stockwell is the as yet unnamed Shepherd's Bush bomb suspect.
A second picture of the man, wearing a white vest and on board a bus heading south after the failed bombing attempt of 21 July, was released on Wednesday night, shortly after the raid, during which three women were arrested and three children taken away.
The man has not been seen at the property since the first suicide bomb attacks and his wife, who wore traditional Islamic dress, and their three young children, were said to have spent most of their time inside. The man appeared to have been unemployed, but had recently told a neighbour he was selling fruit and vegetables at Brixton market.
Residents at the block of flats said the man had recently changed his appearance and become more devout. Dean Pennant, 35, who lives opposite the flat said: "He's lived here for at least six years with a woman and three kids. She recently had another baby. He used to wear normal Western dress, but over the past year he started wearing traditional long robes and grew a beard. He used to pray - whenever I saw him, he told me he was going to mosque."
The man is said to have regularly received visits from other men wearing Muslim dress. One of them bore a resemblance to Muktar Said Ibrahim, the 27-year-old Eritrean suspected of having tried to blow himself up on the Number 26 bus in Hackney on 21 July, who is still on the run.
Rosemarie Rennalls, 29, who lives in the block said Ibrahim was "definitely" a visitor to the flat, "I remember him because he's got a very round podgy face, although he had a lot longer beard then," she said. "He wore the same thing they all did - the dress, cap and slippers."
The two-bedroom flat last night remained shrouded in blue tarpaulin as forensic officers continued their search. A blue car in the neighbouring road was also being examined.
In Tooting, nine people were arrested in raids on two flats, one above a kebab takeaway. Scotland Yard sources said that of those arrested - said to include Algerian, Turkish and Iranian men - none were believed to be suspected bombers.
Neighbours saw six people, including an elderly, traditionally dressed Muslim man as well as a blonde and Chinese-looking women, being led away at 10-minute intervals. Locals described how several people, often partially clad, barefoot, wearing plastic cuffs and flanked by two officers, were run down the road. Police completed their search after several hours and later four women were allowed to return to the house.
At the takeway, Ahmet Ali, the owner, arrived for work and attempted to open up, despite a police cordon, only to discover the people arrested were his three night staff - one man in his forties who had worked for him for nine years and two in their twenties who had been there just two months.
"It is very shocking. They seem like normal, nice people," said his wife.Reuse content