Nerve Gas attack

According to the Sunday times the police have foiled a plan by al-Qaeda to attack the House of Commons with a nerve gas.

Plans for the attack, along with the plans to attack the tube were discovered in coded emails on computers seized from suspected terrorists in Britain and Pakistan.
The emails which contained details of an attack on the House of Commons using sarin gas were decoded with the help of an informant.

Sarin gas

500 times as toxic as cyanide. It's current use is predominantly as a military chemical nerve agent. Sarin emits very toxic fumes of fluoride and phosphorus oxides when heated to decomposition or reacted with steam. At room temperature, sarin takes a liquid form, but it evaporates quickly into a gas. Sarin acts and dissipates more quickly than other nerve agents.
Death may occur within one to 10 minutes of inhalation exposure to even a small amount of sarin.

Saddam Hussein used sarin along with mustard gas on the Kurds in northern Iraq during Anfal and in 1988 in Halabja.
There are also reports that al-Qaeda have tried to make chemical weapons, including sarin and VX.