The head of MI5 during the 2005 London bombings today said she feared at the time the service would not be able to cope with continuing terror attacks.
Baroness Eliza Manningham-Buller said following the 7/7 suicide bombings and the failed attacks a fortnight later, she suspected terrorists could try to bomb the capital on a regular basis.
She said investigators did not immediately know the attack on the capital's public transport system - which killed 52 people - was carried out by suicide bombers.
Lady Manningham-Buller, director general of Britain's domestic intelligence agency between 2002 and 2007, was speaking ahead of the publication of MI5's first official history today.
In a BBC interview she said: "My recollection of 7/7 was a feeling of 'It's happened', what we half expected would, what we had prepared for, what we had trained for, so it was a bad day for everybody.
"But the service started immediately doing all the things that it knew it had to do.
"In the early days we did not know it was a suicide bombing until the forensics began to come through. So at the beginning we were trying to support the police in possibly finding the team who had done it, who for all we knew, at that stage were still alive and capable of mounting another attack."
She recalled the failed attacks on July 21: "For me it was worse, although nobody died, although it was an unsuccessful attack, I had that feeling that if this is going to happen every fortnight how are we going to cope with this?
"How are the police going to cope?
"It was the apprehension that this might be a continuing problem."
Lady Manningham-Buller also defended MI5's interrogation of suspects, saying its agents operated to the "highest ethical standards".
She wanted to speak out on the subject of torture but could not at the moment while cases involving the alleged mistreatment of British terror suspects were before the courts.