The operation envisages prolonged bombardment of Iraqi defences followed by the landing of light brigades of airborne troops once resistance has collapsed. The air option is attractive to London because it will keep British involvement to a minimum in a war that is bound to provoke huge public protest.

The disquiet over Tony Blair's backing for George Bush on Iraq was in evidence again yesterday when church figures, including the Archbishop of Canterbury-designate, Dr Rowan Williams, presented a 3,000-name petition to Downing Street questioning the moral and legal validity of a war.

Hans Blix, the United Nations chief weapons inspector, said there was little new in the latest offer from Saddam Hussein to hold talks. " It's rather tricky diplomatic language. But everything indicates, when you read it very carefully, that it is the same sort of set-up as we have had during three rounds of discussions in New York in the spring."

Kofi Annan, the UN secretary general, wrote to Baghdad yesterday in an attempt to clarify the offer, insisting Iraq must first agree to the return of UN weapons inspectors.

During a visit to Jordan, Naji Sabri, the Iraqi Foreign Minister, said: "We will chop off the head of anyone whose hand reaches our border. We will defend ourselves, shrines and land with all faith and determination against colonialist greed and against those who attack us."

The Americans would need little help from other countries to launch aerial attacks. As in the Afghan campaign, the British role is likely to be limited to providing bases, air refuelling and supplying reconnaissance aircraft. The air war has the advantage of avoiding the political problem of America and Britain needing to find bases for troops in countries neighbouring Iraq.

Of countries with bases, Jordan has become one of the most vociferous lobbyists against a war, the Saudi rulers fear internal turmoil if they allow the country to be used as a launchpad, and Kuwait and Turkey, expected to be supportive, are proving lukewarm to the prospect of hosting hundreds of thousands of American troops. At the end of a successful bombing campaign, a force of about 70,000 could be sent in by sea and air, without the need for large local bases, the sources suggest.

American and British planes patrol Iraq regularly, enforcing the no-fly zones. As well as operating from bases in Kuwait, Bahrain and Qatar, allied planes would be able to fly from the British air base at Diego Garcia in the Indian Ocean andfrom America. In addition, the Pentagon has bases in a number of the former Soviet central Asian republics.

The most widely discussed plan so far envisages a "son of Gulf War" land, air and sea campaign with a force of 250,000 troops going into Iraq from Jordan, Kuwait and Turkey. Generals have warned that about 10 per cent of the force might end up as casualties in a land war.

Prolonged air strikes would minimise losses. Iraqi air defences are believed to be in a poor state, and the Iraqi air force, which hardly flew during the Gulf War, has since lost a fifth of its planes. As few as 30 per cent are said to be serviceable. At the same time, American bombing capability has improved vastly. During the Gulf War only one in 10 of the bombs was a "smart bomb". Now that figure is nine out of 10. Manufacturers are working hard to produce a range of specialised bombs and unmanned recconaissance aircraft.