A TERRORIST attack on a European city using nerve gas, germ warfare, radioactive or nuclear weapons to inflict mass casualties is inevitable, the head of MI5 said yesterday.

Eliza Manningham-Buller, who became director-general of the domestic intelligence agency last October, told a conference in London that ''renegade scientists'' had already given terrorist groups the information to build such devices.

''We are faced with a realistic possibility of a form of attack that could include chemical, biological, radiological or nuclear. It is only a matter of time before a crude version of one of these is launched on a western city,'' she said in her first on-the-record briefing since taking up her (pounds) 150,000-a-year post.

She said September 11 was a watershed. Intelligence knew something was going to happen, but not the nature, or size, of the attack.

She conceded that no intelligence service would ever be able to present a complete picture of terrorist intentions and targets, particularly now when the threat was global and the complexity of alliances involving networks like al Qaeda difficult to pin down. ''There was plenty of intelligence before September 11 that something terrible was going to happen.

''That was reflected in the dossier on Osama bin Laden that the government published shortly afterwards. But the nature of the attack was unknown,'' she added.

''Intelligence never tells you everything. In this case, when it's a global threat, it tends to be patchy and incomplete. Sep-tember 11 was a watershed. We were shocked by the sheer scale of the devastation, which was beyond our imagination.''

Ms Manningham-Buller, 54, said about 150 new pieces of intelligence about possible terrorist threats were received every week at MI5's joint terrorism analysis centre in London.

The willingness of networks like al Qaeda to strike soft targets and to use suicide bombers made it difficult to stop, despite increased international co-operation and greater resources. The positive note was that Britain had ''unrivalled expertise'' in combating terrorism.

Britain was placed on its second-highest security alert last month shortly after Ms Manningham-Buller and the CIA director, George Tenet, met for a private summit. Within days, the Houses of Parliament and other vulnerable public buildings in London were ringed with concrete barriers.

Al Qaeda still remained an organisation capable of deadly attacks. It was ''the first truly global terrorist threat''. Recent bomb attacks in Riyadh and Casablanca proved that it and other groups were still active and, potentially, potent.

''The threat from international terrorism is with us for a good long time. If this is a war that can be won, it is not going to be won soon,'' she added. ''Breaking the link between terrorism and religious ideology is difficult.''

She joined MI5 in 1974. Under her command, the agency has been given funding and permission to increase its staff by 25% to 2400 to counter international terrorism and organised crime syndicates.