British police defused a second car bomb on Friday, hours after finding a first explosive device, the city's anti-terrorist chief said, adding that the two were clearly linked and calling the development "troubling."
The find raised the spectre of possible Al-Qaeda terrorism returning to the British capital, two days after a new government took power and a week before the second anniversary of the city's July 7, 2005 suicide bombings, which killed 52.
"This like the first device was potentially viable and was made safe by the explosives officers," said Peter Clarke, the Metropolitan Police anti-terrorism chief.
Like the first car, the second one was a Mercedes which was found parked near the Haymarket entertainment district of central London.
But while the first one was discovered by security staff who called in police, the second was taken to an underground car pound near Park Lane, where the explosives were discovered after police were alerted later in the day.
"There was a considerable anount of fuel and gas canisters. As in the first vehicle, there was also a substantial quantity of nails," said Clarke.
Professor Anthony Glees of Brunel University, an intelligence expert at Brunel University, said the apparent simultaneous bombing attempt pointed to Al-Qaeda.
"There even more evidence now that this is an Al-Qaeda plot. It's an Al-Qaeda memo to Gordon Brown," he said, referring to the new prime minister who succeeded Tony Blair on Wednesday.
"It's an Al-Qaeda memo to the British public, that they are determined to carry on...their attack on ordinary British people," he said.
Speaking before the second device was found, Brown said the alert was a fresh warning of the threat faced by London, which next week marks the second anniversary of the July 7, 2005 suicide attacks which killed 52 people and the four bombers.
Speaking after the first bomb was found, Clarke would not speculate on who was behind the device -- comprising gas cylinders, petrol and nails -- found outside a nightclub on Haymarket near Piccadilly Circus.
But he said: "Even at this stage it is obvious that if this device had detonated, there could have been significant injury or loss of life."
Rajeshree Patel, who was in the Tiger Tiger club when it was evacuated, told BBC television: "I think there would have been a lot of fatalities. There were approximately 500 people inside Tiger Tiger at the time."
Clarke said police had no warning of an attack and it was unclear if the nightclub was the target but there were similarities with previous plots.
Members of an Islamist-inspired gang were jailed for life earlier this year after plotting to attack a number of high-profile British targets, including London's Ministry of Sound nightclub.
And a Muslim convert was put behind bars for 30 years here last November for plotting devastating attacks in London and New York, including a plan to detonate limousines packed with explosives at key landmarks.
A security source quoted by Britain's Press Association news agency said it was "entirely possible" the latest incident had overseas links as insurgents in Iraq had used similar methods, but they were keeping an open mind.
The alerts provided a baptism of fire not just for Brown, but also for his Home Secretary Jacqui Smith, who was less than 24 hours into the role.
She met Brown in Downing Street and afterwards called for public vigilance.
"We are currently facing the most serious and sustained threat to our security from international terrorism," she said.
Smith earlier chaired a meeting of the government's emergency contigencies committee, COBRA, and reported to Brown's senior ministers at an extended cabinet meeting.
The first bomb was discovered by chance -- Clarke said an ambulance crew treating a person at Tiger Tiger called in police explosives experts after noticing a metallic green Mercedes car giving off smoke just before 2.00 am (0100 GMT).
Inside they found "significant quantities" of petrol and a "large number" of nails, he added.
Police sources said there was as much as 60 litres of petrol on the back seat of the car and in the boot (trunk).
A hunt was on for the driver with detectives expected to scour footage from closed circuit television cameras in streets surrounding Haymarket, which is busy with revellers into the early hours of the morning.