Using information from emails found on the computers of suspected terrorists, British police believe that they have foiled an Al Qaeda gas attack attack on the British parliament.

Planning for the attack apparently began last year:

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

A senior officer said the scheme had led to the intervention of Eliza Manningham-Buller (search), 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.”

A great stroke of luck it was that those emails were discovered.