Britain faces a threat from suicide bombers, with a terror attack almost inevitable, one of the country's senior police officer has said.
"The suicide threat is there and we have to take account of that," Sir John Stevens, who runs London's police force, said in an interview.
Stevens also said al Qaeda was recruiting in Britain and that a real threat existed of chemical and biological attacks.
"We are at the highest level of alert, I think, that we have ever been," he said.
Since the September 11, 2001, attacks on the United States, Washington's top ally London has been considered a prime target.
There has never been a suicide bomb attack in Britain but the threat took "a quantum leap," Stevens said, after a series of deadly attacks this year on targets stretching from Saudi Arabia to Morocco to Russia.
In May, two Britons took part in a suicide bombing in Israel that marked the first of its kind by foreigners sympathetic to the Palestinian uprising.
"That quantum leap is all about people prepared to give their lives in relation to their causes," Stevens added
While there was no specific intelligence to point to an imminent attack, Stevens said it was only a matter of time and that he would not hesitate to put troops on the capital's streets if necessary.
"The prime minister (Tony Blair) says an attack's inevitable and...I think that's probably right," he said
"When it happens, we have to have everything in place."
In May, large concrete blocks were erected around parliament buildings to protect them from suicide car bombers and in February, tanks and armoured cars were deployed around London's Heathrow airport after intelligence pointed to a threatened missile attack.
In a sign of how the risk has escalated, Stevens said anti-terror officers were now working up to three times harder than during the height of the troubles.