The Headquarters of MI6 - the British government's secret intelligence service - was the subject of a major police and anti-terrorist investigation this morning after at least one explosion in the Vauxhall area of south London late last night.
At 3.15 this morning the head of the UK's anti-terrorist squad, Mr Alan Fry, said "some form of small missile" had been fired at the building. It had caused minimal damage, he said. Witnesses spoke of two explosions, but Mr Fry would not confirm more than one. He would not speculate on the type of device involved, but said he believed that a mortar would have caused more damage.
There was speculation about a possible dissident republican attack on the British capital on the eve of the crucial South Antrim by-election, but there was no confirmation early this morning of who was responsible. Mr Fry said he had to "keep in mind" the possibility that dissident Republicans were involved, but it was too early to speculate and he would not rule out the possible involvement of other terrorist groups.
There were no casualties reported following the explosions.
Police were called to the scene at 9.45 p.m. last night, according to a Scotland Yard spokesman, who said there had been some damage to the eighth floor of the building.
All the roads around the distinctive hi-tech cream-and-green structure, which is a mile from the Houses of Parliament, were sealed off as the investigation into the explosion began. Surrounding roads and railway lines will be closed off this morning while the investigation proceeds. "Painstaking forensic examination " of the building and the surrounding area will be undertaken, according to Mr Fry.
Eyewitnesses reported seeing smoke over the top of the building. One eyewitness, Mr Andrew Pierce, was driving home with his girlfriend when he saw a flash of light coming from the top of the MI6 building.
He said: "It looked as if it was internal, from the centre of the building - the flash of light was from the top of the building. It was followed by a large bang and the ground seemed to shake, and my girlfriend said `I think that was a bomb'."
The London Ambulance Service, which was called to the scene just before 9.50 p.m., said there were no reported casualties and that it was "safe to assume there are none".
Scotland Yard was not commenting early this morning on who might have been responsible, although the involvement of the anti-terrorist branch was a clear indication of how seriously the incident was being taken.
Dissident republicans have been linked to two previous bomb attacks in London this year. The "Real IRA" was blamed for an explosion under Hammersmith Bridge on June 1st. Nobody was harmed in the incident but there was widespread disruption to early-morning commuter traffic.
Dissidents are believed to have planted a second device on a west-London rail line on July 19th. The 1/2 kg bomb, placed close to Ealing Broadway station, failed to detonate but brought London's rail network to a near standstill for several hours.
Security sources have expressed concern recently that the "Real IRA", the group responsible for the Omagh bomb, was intent on carrying out attacks on sensitive targets in London.
Republican dissidents have recently perfected a mortar after a year of experimenting, and two weeks ago a missile landed in the car park of Armagh RUC station. The mortar was fired from a van parked more than 50 m from the station.