Security sources say that the two bombs, which came 14 hours after the Bishopsgate bomb in the City of London, were an attempt by the IRA to demonstrate to police that it can operate with freedom in the capital.
Police praised the two taxi-drivers who alerted the emergency services after the terrorists - who had ordered them at gunpoint to drive to Downing Street and Scotland Yard - abandoned them, leaving the bombs on the back seats of the vehicles.
According to Scotland Yard, the first mini-cab was hired by a man who walked into Acropolis Cabs in Seven Sisters Road, in the Finsbury Park area of north London.
The man asked to be driven to Tottenham Court Road in central London. Outside the office, the man and the driver were joined by another man, carrying a bag over his shoulder, who got into the back.
About 400 yards from the cab office, the man in the front seat threatened the driver with a small handgun. In an Irish accent, he told him: 'There is a bomb on the back seat which will be detonated by remote control. Drive it on to 10 Downing Street. We will get out here but we will be following you in another car.'
The driver headed towards central London but by the time he reached the King's Cross area, he realised he was not being followed. The man stopped, alerted police and the area was cleared by the time the device detonated shortly before midnight.
By this time, a second man had walked into the same mini- cab office and asked to be taken to central London. A few hundred yards down the road, he asked the cab to stop and pick up another man. They told the driver they were armed and there was a bomb on the back seat; he was ordered to drive to Scotland Yard and both men got out of the vehicle and ran away.
The driver abandoned his vehicle and asked a bus driver to radio for police. The area was evacuated before the bomb exploded just one minute before the King's Cross device detonated.
Police said that the first suspect to enter the mini-cab office was white, 24, 5ft 7in, of slim athletic build, with blond, short hair and wore jeans and trainers.
His companion with the bag was white, 45, 5ft 10in, and wore a black cricket cap, an overcoat and black trousers.
The second man to enter the mini-cab office was white, 20 to 25, about 5ft 7in tall, clean shaven and wearing a black baseball cap, dark glasses, a dark jacket and jeans.
The man he picked up was described as white, of medium build and wearing a black baseball cap.
Ben Isaac, who runs the cab company, said: 'Both of the drivers are deep in shock and are just trying to cope with what has happened to them.
'One of the chaps is a family man with three kids and he just doesn't know what he is going to do,' he said.