According to the Times of London, British intelligence cracked Al Qaeda codes and believes that it prevented a chemical attack (involving sarin) on Parliament (tip to Hewitt):

SCOTLAND YARD believes it has thwarted an Al-Qaeda gas attack aimed at ministers and MPs in parliament. The plot, hatched last year, is understood to have been discovered in coded e-mails on computers seized from terror suspects in Britain and Pakistan. Police and MI5 then identified an Al-Qaeda cell that had carried out extensive research and video-recorded reconnaissance missions in preparation for the attack.

The encrypted e-mails are said to have been decoded with the help of an Al-Qaeda "supergrass". By revealing the terrorists' code he was also able to help MI5 and GCHQ, the government's eavesdropping centre at Cheltenham, to crack several more plots.

The discovery of the suspected Commons nerve gas plot was behind the decision to increase security around parliament this summer.

A senior officer said that the scheme had led to the intervention of Eliza Manningham-Buller, head of MI5, to assess parliament's security.

The operation to deter the sarin gas attack is referred to in an internal police document obtained by The Sunday Times.

It is a minute of a meeting of senior police officers held last month at Specialist Operations 17 (SO17), the unit responsible for protecting parliament, and reveals that the team were waiting to be briefed on the plot.

This weekend a senior officer disclosed that the thwarted plot mentioned in the document involved a gas or chemical "dirty bomb" attack against parliament. "The House of Commons was one of their targets as well as the Tube," he said.

"They were planning to use chemicals, a dirty bomb and sarin gas. They looked at all sorts of ways of delivering it."