Britain was drawn into the controversy surrounding America's use of white phosphorus in last year's battle for Fallujah today when the Defence Secretary conceded that British forces had also used the substance during military operations in Iraq.

But John Reid said that the Army had used the incendiary metal only to provide a smokescreen for British troops rather than deploying it as a weapon directly against enemy personnel - as the Pentagon admits to having done in Fallujah.

"I think the Americans have to answer the questions that are put to them on that," Dr Reid told the BBC on a visit to Germany. "I can only speak for the British and we only use it as a smokescreen, to give cover to our troops."

Legally, the difference is a crucial one. Critics say that the American military may have breached international conventions through their use of the substance to flush out insurgents from the rebel stronghold of Fallujah.

A wax-like metal that combusts spontaneously when exposed to oxygen, white phosphorus can burn right through skin to the bone. In Vietnam, where it was regularly used alongside Napalm, it was known as Willie Pete.

Its use in Fallujah was highlighted by a documentary on the Italian state television channel RAI this month which alleged that American use of the weapon had caused injuries and deaths not just to insurgents but to civilians. One former US soldier told RAI that he had seen the "burnt bodies of women and children".

After an embarrassing denial yesterday by the new US Ambassador to Britain, Robert Tuttle, that white phosphorus had been used in Iraq, a Pentagon spokesman confirmed its use but said that it was not a banned weapon.

"White phosphorus is a conventional weapon. It is not a chemical weapon," said the spokesman, Lieutenant-Colonel Barry Venable. "They are not outlawed or illegal.

"We use them primarily as obscurants, for smokescreens or target-making in some cases. However, it is an incendiary weapon and may be used against enemy combatants."

Protocol III of the UN's 1980 Convention on Certain Conventional Weapons expressly forbids the use of white phosphorus against civilian targets or military targets in civilian areas. The United States has not signed up to that protocol, although Britain has.

But America is a signatory to the Chemical Weapons Convention, which it ratified in 1997, and that agreement forbids the use of any substance to kill or harm either soldiers or civilians if it is being used mostly for its toxicity.

Although the Pentagon spokesman described it as an "incendiary weapon", white phosphorus has clear chemical effects on its human targets - melting away the skin of its victims and burning quickly into the tissue, especially on exposed parts of the body like the face and hands.

A US Army handbook published in 1999 states clearly that the use of white phosphorus burster bombs against enemy personnel is "against the law of land warfare" and the US State Department clearly denied last year that any such weapons were being deployed in Iraq.

But an article in an internal US Army magazine provided incontrovertible evidence that such bombs were used against human targets in Fallujah and the Pentagon appears to have changed its mind on the precise legal status of the substance.

That article, in the March/April edition of Field Artillery was written by three soldiers who were involved in the battle for Fallujah last November.

They wrote: "WP proved to be an effective and versatile munition. We used it for screening missions at two breeches and, later in the fight, as a potent psychological weapon against the insurgents in trench lines and spider holes when we could not get effects on them with HE (high explosives).

"We fired 'shake and bake' missions at the insurgents, using WP to flush them out and HE to take them out."

Professor Paul Rodgers, an international security expert at Bradford University, told Times Online: "The real difficulty is that it depends on the way it is used - and it depends very narrowly on whether you define the action of white phosphorus as chemical or incendiary.

"It's very much a grey area."

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