Iraq's biological arsenal could do the most damage if it were used to retaliate immediately against a US invasion rather than in later stages.

Although US troops are being vaccinated against anthrax and smallpox and have protective gear, a biological attack cannot be detected until after exposure.

Even if a biological attack did not kill US troops, it could still kill many civilians, slow a US advance and strain medical capabilities.

'The most frightening thing is Iraq's biological programme,' said Mr David Kay, a former UN chief weapons inspector.

'Even in my inspection days, it was the programme we knew the least about.'

What inspectors eventually learned was disturbing.

After the 1995 defection of Mr Saddam's son-in-law, who ran the germ weapons programme, Iraq acknowledged brewing thousands of gallons of deadly germs, toxins and loading them in bombs, missile warheads and rockets.

In late 1998, frustrated by Iraq's refusal to cooperate, the inspectors withdrew shortly before the US and Britain began 'Operation Desert Fox'.

Iraq claimed it destroyed all its biological weapons, but UN inspectors concluded in 1999 this was probably a lie.

US officials say Iraq's latest weapons declaration does not clear up discrepancies.

'Before the inspectors were forced to leave Iraq, they concluded that Iraq could have produced 26,000 litres of anthrax. That is three times the amount Iraq had declared,' Secretary of State Colin Powell said recently.

'Yet the Iraqi declaration is silent on this stockpile, which alone would be enough to kill several million people.'

» Germ warfare is Iraq's secret trump card - DEC 29, 2002