Tony Blair, his key officials and senior ministers will be moved to an “alternative seat of government” with nuclear bunkers, top security but also good communications, The Times has learnt.

The disclosure came as David Blunkett announced that the Government would soon stage a huge exercise “simulating a catastrophic incident” in the capital.

It later emerged that that would involve a mock chemical weapons attack on the London Underground near Bank station in the City on Sunday, March 23.

The stark prospect of a Britain at war on the home as well as international front was painted as the civil emergency plans were unveiled and American B52 bombers landed at RAF Fairford in Gloucestershire as part of the build-up for conflict with Iraq.

The bombers, used in the first Gulf conflict and in Kosovo, flew in as coalition aircraft stepped up their activity in the no-fly zones over Iraq, including claimed strikes against surface-to-surface missile units. It prompted the Conservatives to declare that the opening shots of the second Gulf war had been fired.

Until recently only 21 RAF aircraft were involved in what is codenamed Operation Resinate, the longstanding mission to protect the Kurds in the north and Shias in the south from Iraqi attacks. Now the figure has reached about 60 and will soon be 100.

At the same time, Britain and America intensified diplomatic efforts to win UN Security Council support for a second resolution that would trigger conflict. Iraq made that more difficult by declaring that it would submit a report next week on its VX nerve gas and anthrax stocks.

Jacques Chirac hardened France’s opposition to the resolution, making talks in London today with the Russian Foreign Minister, Igor Ivanov, even more vital. The discussions between Mr Ivanov and Jack Straw will take place at 10 Downing Street rather than the Foreign Office, and Mr Blair will join them.

At Westminster the sense that war was inevitable was hardening. The Times has learnt that a group of junior ministers who are opposed to military action without a UN mandate are to meet informally this week to discuss tactics.

A handful are said to be ready to consider resigning if Britain and the US openly flout the will of the UN. But Labour’s senior business managers say that these are the only circumstances in which ministers would quit. Cabinet sources say there would be no resignations if the second resolution were secured, and that there were unlikely to be any if the resolution was the subject of an “unreasonable veto”.

However, they admit that some ministers would feel unable to stay in the Government if Britain and the US failed to get the nine required votes in the Security Council or if the resolution were vetoed by a country other than France.

Robin Cook and Clare Short are seen as the most likely Cabinet ministers to go in that situation and several others in the lower ranks are understood to have indicated to the whips that they would have to consider their positions.

The London terrorism exercise is being staged when the Underground is quiet and no members of the public will be allowed on the Tube train involved. However, officials said that rescue teams would be unaware of the extent of the simulated incident or what precisely had happened.

Two separate exercises are also being planned to test the country’s flood defences and the national gas supply, the Home Secretary said.

He took parts of Whitehall by surprise and caused confusion within different government departments. The Home Office said that though the word “catastrophic” had been used, people were getting overexcited about what was planned. One official said that the incident would be a “tabletop” exercise to test the response and co-ordination of the emergency services and national and local government. Another official said: “It will be a live exercise. The public will be aware that something is going one. It will be visible.”

Later Mr Blunkett caused further confusion over the Government's policy towards providing the public with smallpox vaccinations. Oliver Letwin, the Shadow Home Secretary, challenged the Government’s policy saying: “I cannot for the life of me understand how (smallpox vaccination) is not made available to anyone who wishes to use it.” Mr Blunkett replied: “I don't believe there is a problem with people approaching their own practitioner for a smallpox vaccination. If there is a difficulty I will happily approach the Secretary of State for Health.”

One official said later: “It is astonishing. The Home Secretary has just undermined our line on smallpox. The public will be queueing outside surgeries to get jabs.” A Health Department spokeswoman said: “Mass vaccination is not warranted where there is little or no risk of exposure.”

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