American investigators probing anthrax outbreaks in Florida and New York believe they have all the hallmarks of a terrorist attack - and have named Iraq as prime suspect as the source of the deadly spores.

Their inquiries are adding to what US hawks say is a growing mass of evidence that Saddam Hussein was involved, possibly indirectly, with the 11 September hijackers.

If investigators' fears are confirmed - and sceptics fear American hawks could be publicising the claim to press their case for strikes against Iraq - the pressure now building among senior Pentagon and White House officials in Washington for an attack may become irresistible.

Plans have been discussed among Pentagon strategists for US air strike support for armed insurrections against Saddam by rebel Kurds in the north and Shia Muslims in the south with a promise of American ground troops to protect the oilfields of Basra.

Contact has already been made with an Iraqi opposition group based in London with a view to installing its members as a future government in Baghdad.

Leading US intelligence sources, involved with both the CIA and the Defence Department, told The Observer that the 'giveaway' which suggests a state sponsor for the anthrax cases is that the victims in Florida were afflicted with the airborne form of the disease.

'Making anthrax, on its own, isn't so difficult,' one senior US intelligence source said. 'But it only begins to become effective as a biological weapon if they can be made the right size to breathe in. If you can't get airborne infectivity, you can't use it as a weapon. That is extremely difficult. There is very little leeway. Most spores are either too big to be suspended in air, or too small to lodge on the lining of the lungs.'

As claims about an Iraqi link grew, senior health officials in Britain revealed they warned all the country's GPs last week to be vigilant about the disease. 'I think we have to be prepared to think the unthinkable,' said the Government's Chief Medical Officer, Dr Liam Donaldson. The Department of Health confirmed the Government is conducting an urgent review of Britain's ability to cope with chemical or biological attacks.

It also emerged last night that three people who worked in the Florida buildings at the centre of anthrax scares are now in the UK and undergoing tests for the disease. And in America a letter sent from Malaysia to a Microsoft office was found to contain traces of anthrax.

In liquid form, anthrax is useless - droplets would fall to the ground, rather than staying suspended in the air to be breathed by victims. Making powder needs repeated washings in huge centrifuges, followed by intensive drying, which requires sealed environments. The technology would cost millions.

US intelligence believes Iraq has the technology and supplies of anthrax suitable for terrorist use. 'They aren't making this stuff in caves in Afghanistan,' the CIA source said. 'This is prima facie evidence of the involvement of a state intelligence agency. Maybe Iran has the capability. But it doesn't look likely politically. That leaves Iraq.'

Scientists investigating the attacks say the bacteria used is similar to the 'Ames strain' of anthrax originally cultivated at Iowa State University in the 1950s and later given to labs throughout the world, including Iraq.

According to sources in the Bush administration, investigators are talking to Egyptian authorities who say members of the al-Qaida network, detained and interrogated in Cairo, had obtained phials of anthrax in the Czech Republic.

Last autumn Mohamed Atta is said by US intelligence officials to have met in Prague an agent from Iraqi intelligence called Ahmed Samir al-Ahani, a former consul later expelled by the Czechs for activities not compatible with his diplomatic mission.

The Czechs are also examining the possibility that Atta met a former director of Saddam's external secret services, Farouk Hijazi, at a second meeting in the spring. Hijazi is known to have met Bin Laden.

It was confirmed yesterday that Jim Woolsey, CIA director from 1993 to 1996, recently visited London on behalf of the hawkish Defence Department to 'firm up' other evidence of Iraqi involvement in 11 September.

Some observers fear linking Saddam to the terrorist attacks is part of an agenda being driven by US hawks eager to broaden the war to include Iraq, a move being resisted by the British government.

The hawks winning the ear of President Bush is assembled around Defence Secretary Donald Rumsfeld, his deputy Paul Wolfowitz, and a think tank, the Defence Policy Advisory Board, dubbed the 'Wolfowitz cabal'.

Their strategy to target Iraq was hammered out at a two-day seminar in September, of which the dovish Secretary of State Colin Powell had no knowledge.

The result was a letter to President Bush urging the removal of Saddam as a precondition to the war. 'Failure to undertake such an effort,' it said, 'will constitute a decisive surrender in the war against terrorism'.

In a swipe at Powell's premium on coalition-building, it continues: 'coalition building has run amok. The point about a coalition is "can it achieve the right purpose?" not "can you get a lot of members?"'

Administration officials close to the group told The Observer : 'We see this war as one against the virus of terrorism. If you have bone marrow cancer, it's not enough to just cut off the patient's foot. You have to do the complete course of chemotherapy. And if that means embarking on the next Hundred Years' War, that's what we're doing.'