The hole in the supposed NYC subway cyanide plot

But perhaps the most chilling claim in the book is that al-Qa'eda has mastered how to make a simple device to release hydrogen cyanide into enclosed spaces.

Al-Qa'eda agents were 45 days from attempting a cyanide attack on the New York subway before it was vetoed by Ayman al-Zawahiri, Osama bin Laden's deputy, the book claims.

Intelligence officials and senior Bush aides speculate in the book that al-Zawahiri feared it would not cause enough carnage as he wanted the next attack on the US mainland to eclipse the September 11 attacks. But it is unclear how accurate the intelligence is.

American agents first learned about the supposed plot in February 2003 from the contents of a laptop belonging to a Bahraini jihadist captured by the Saudis. It contained encrypted plans for building a simple delivery system for hydrogen cyanide gas - a canister with two chambers separated by a seal. Sodium cyanide would be in one and a hydrogen product in the other.

Once the seal was broken, by a fuse that could be triggered by a mobile phone, the two would form the gas, unleashing one of the great nightmares of public transport officials.

An al-Qa'eda cell planned to release cyanide gas into the New York subway before the Iraq war, but the plot was called off by Osama bin Laden's deputy, it was claimed yesterday.