The head of Britain's domestic security services, MI5, has warned that civil liberties across the world may have to be sacrificed to prevent future terrorist attacks.
In a speech to security officials in the Netherlands earlier this month and available on the MI5 Web site yesterday, Dame Eliza Manningham-Buller said the deaths of 52 people in the July 7 London bombings had shocked everyone, "and my service and the police were disappointed that we had not been able to prevent them."
British authorities "value civil liberties and wish to do nothing to damage these hard fought rights," she said.
"But the world has changed, and there needs to be a debate on whether some erosion of what we all value may be necessary to improve the chances of our citizens not being blown apart as they go about their daily lives," she said.
Manningham-Buller said difficult decisions often needed to be made on the basis of intelligence that was "fragmentary and difficult to interpret."
The main dilemma, she said, was how to protect citizens within the rule of law when "fragile" intelligence did not amount to the kind of clear cut evidence that would support a criminal trial.
"We can believe, correctly, that a terrorist atrocity is being planned, but those arrested by the police have to be released as the plan is too embryonic, too vague to lead to charges and possibly convictions," she said.
"Furthermore the intelligence may be highly sensitive, and its exposure would be very damaging as revealing either the source or our capability," she said.
Her remarks echo those of other top British officials.
On Wednesday, Home Secretary Charles Clarke warned that EU states and judges needed to accept an erosion in freedoms in the wake of the London attacks and last year's Madrid train bombings.
The government is frustrated that tough measures, such as deporting al-Qaeda suspects or extremist clerics who could inspire terrorism, were being hindered by the courts because of civil rights concerns.
Last year, the UK's most senior court threw out a security law allowing police to detain foreign terrorist suspects indefinitely without trial saying the legislation breached basic human rights.
The government said there needed to be a redress in the balance of the rights of suspects and defendants and those of ordinary citizens, adding that the European Human Rights Convention might have to be amended if necessary.
However, the wife of an Algerian terrorist suspect who is in custody facing deportation from Britain and known only as "G", accused authorities of infringing her husband's rights by locking him up on the basis of secret evidence.
She said "G" would be tortured on his return to Algeria.
"If they bring him to justice with the legal judgement I will accept that. But like this, no," she told BBC radio.