THE people responsible for the audacious assault on the heart of Britain's international spying network yesterday remained as secretive as the agents working on at the damaged MI6 headquarters.

Scotland Yard last night said that they had found part of a rocket launcher in a park in Vauxhall Cross, but a spokeswoman said it was not known if the launcher had been fired from that spot.

No-one has claimed responsibility for the explosions, but security sources believe Irish republican dissidents from the splinter group the Real IRA are responsible.

The group, which is opposed to the Northern Ireland peace process, was behind the Omagh bomb that killed 29 in August, 1988, and is thought to have launched two previous explosive attacks in London this year.

Security was tightened throughout the capital last night, amid fears of an escalating campaign. One source said of the latest attack: ''This is clearly an organisation which is becoming more and more aggressive and confident of what it can achieve and sustain. It has the capacity to strike right at the heart of London. . . . it's a force to be reckoned with.''

Police scoured an area up to 500yds around the #200m headquarters on the south bank of the Thames, with railway lines running into Waterloo station receiving particular attention from forensic scientists. The cordon was lifted at 5pm, easing traffic chaos.

''This afternoon, while carrying out a detailed search of the Spring Gardens area of Vauxhall Cross, Metropolitan Police service officers recovered part of a rocket-propelled grenade launcher,'' the police spokeswoman said.

She added: ''This is a dangerous military weapon. At this stage it is not possible to say if the launcher was fired at the location where it was found but detailed inquiries continue. The launcher is now being examined by experts. This will enable us to determine precisely what type of launcher it is.''

Rocket-propelled grenades have been found in Ireland, both north and south; are available in Russia and the former Yugoslavia; and are also used by Middle Eastern terrorists.

Metropolitan Police deputy assistant commissioner Alan Fry said there had been no warning or claims of responsibility for the attack, made from a range of between 200 and 500 metres.

Eighth-floor damage was slight: a window, two panels, and section of metal frame. But the embarrassment caused by the security breach is huge, as is the propaganda coup to the group responsible.

Security specialist Stephen Dorril said: ''It may have been the case that they got successful, the right building, and the headlines they were looking for - but whether they could continue it I don't know.''

Terrorism expert Mike Yardley said the device was probably a rocket-propelled grenade that can be concealed beneath an anorak.

He thought a splinter Republican organisation likely to have been responsible, given the current state of the Northern Ireland peace accord.

Although coded warnings usually proceed such bombings, the rocket launch was late at night - designed for effect rather than to create casualties.

Mr Fry said it was too early to speculate, but added: ''Clearly, we have to keep in mind the capabilities of dissident Irish groups. At this stage we will not be ruling out any group that might see the Secret Intelligence Service as a target.''

The Real IRA is suspected of this summer's bomb attacks on Hammersmith Bridge and a railway line at Ealing Broadway, which prompted warnings of another wave.

Although it called a ceasefire in the wake of Omagh, the group is known to be active again. It would appear an overseas team is now in place, prompting an immediate upgrading of security at other sensitive buildings.

Police yesterday appealed to anyone who saw anything suspicious in the Vauxhall area between 9.30pm and 9.45pm on Wednesday night.

Foreign Secretary Robin Cook, who has responsibility for MI6, said that its chief, Mr Richard Dearlove, had confirmed there was no damage inside the building.

He added: ''I asked him to commend SIS staff for maintaining business as usual. If the intention of the attackers was to disrupt SIS operations, they have failed in their objective.''

Security measures at next week's Labour conference in Brighton are to be tightened, Sussex Police announced.