MINISTERS have drawn up secret plans for the mass evacuation of London in the event of a serious terror threat.

The classified plan, codenamed Operation Sassoon, goes far beyond the anti-terrorism exercise taking place in the capital today.

Confidential documents seen by The Sunday Times show that ministers are prepared to herd Londoners out of the metropolis to ??rest and reception areas?? in the home counties if there is an attack. Even ??high-level intelligence?? showing an attack was imminent could spark a mass evacuation.

The plans also refer to the need for ??medium/long-term rehousing?? if a terrorist attack such as a ??dirty bomb?? rendered parts of London uninhabitable.

The documents formed part of a presentation to the London Resilience Forum at the Cabinet Office in July. They provide a blueprint not just for London but for other big cities.

Last week Sir John Stevens, the Metropolitan Police commissioner, said a terrorist attack on Britain was inevitable and David Blunkett, the home secretary, gave a warning of possible suicide bombings.

Yesterday Nick Raynsford, the minister for London, confirmed the existence of the plans, saying mass evacuation was a last resort.

But he added: ??If there was evidence that a particular area was likely to be subject to an attack and there was no way to prevent this then clearly we would need to take action to protect the public, by evacuation or whatever else was necessary.??

One document lists four reasons for mass evacuation. It cites ??loss of essential services, eg water, electricity; CBRN (chemical, biological, radiological and nuclear) or other terrorist attack; high-level intelligence; voluntary mass evacuation??.

The documents list Heathrow, Canary Wharf, Westminster, Whitehall and the City as possible targets.

The plan details ??how it will be done??. Londoners will initially be directed to safe areas on foot, then ferried outside the M25 motorway to special camps by rail, Underground, buses and private vehicles.

There is also an ominous reference to the need to ??suspend legislation?? in certain circumstances, suggesting that a state of emergency could be declared and the rule of law suspended.

The government is already preparing legislation that would give the police and army new powers if a national emergency is declared. These include giving police the ability to cordon off large residential areas and declare curfews.

Other concerns, which ministers have yet to resolve, are what the documents term ??medium/long-term rehousing?? issues and the horrifying possibility of ??multiple incidents?? ?? several terrorist atrocities happening at once.

It states that ??key partners?? to the mass evacuation will include the airports firm BAA, the Highways Agency, regional government offices, Transport for London and Network Rail as well as the police, firefighters and ambulance services.

A system of text messages, loud hailers, flyers, mail drops, billboards, electronic signs and public address systems will be used to alert people to any threat. According to the documents, early testing of the ??PMB?? ?? police message broadcasting ?? began last month. Its aim is to be able to send 800,000 e-mails an hour when it is ready this month.

One document seeks to calm the otherwise worrying set of images by stating: ??Mass evacuation will always be the last resort.??

The July meeting was chaired by Raynsford and Ken Livingstone, the mayor of London. Also present were high-ranking representatives of the Greater London Authority, Metropolitan and British Transport police, the London coroner??s office, the Home Office terrorism and protection unit, the Cabinet Office civil contingencies secretariat, British Telecom, London Underground, the London First business network and Transport for London.

• The emergency services will today carry out a practice exercise to evacuate Bank Underground station in the City to see how they would cope if there was a chemical attack on the Tube. Television broadcasts will feature prominent warnings to reassure viewers that real terrorists have not struck.

  Police, ambulance and fire crews in chemical suits will seal the station in the most visible drill since September 11. They will practise rescuing and decontaminating commuters, played by 100 police cadets.

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