Lethal explosive can be bought online

It was used in the Bali bombings in 2002, the bombings last November of British targets in Turkey, the World Trade Centre bombing in 1993 and the Oklahoma City bombing in 1995.

It was used in the Bali bombings in 2002, the bombings last November of British targets in Turkey, the World Trade Centre bombing in 1993 and the Oklahoma City bombing in 1995.

Ammonium nitrate - the compound allegedly seized in yesterday's raid - has previously been called al-Qa'ida's explosive of choice, and has been responsible for hundreds of deaths in terrorist attacks worldwide.

The amount of ammonium nitrate found by police yesterday was the same quantity used in the IRA bomb which exploded in London's Docklands at the South Quay rail station which killed two people in 1996. It was also twice the amount reported to have been in each of the four vans used in the Turkey bombings on the British Consulate, HSBC Bank and two synagogues last year.

The second and deadliest explosion in the Bali bombings is believed to have contained only 50 to 150 kilograms of explosive - or about one fifth of the amount of ammonium nitrate found yesterday.

The substance is not that difficult to get hold of. Millions of ton of it are produced each year as fertiliser. Ammonium nitrate - or NH4NO3 - is the basic chemical constituent used in industrial explosives.

Because it is a very stable compound, ammonium nitrate has now widely replaced dynamite and is used mostly in the mining and demolition industry.

It can be obtained quite simply, often without security vetting, over the internet. Dozens of international manufacturers advertise the sale of ammonium nitrate, some promising a 40-day delivery service.

Websites also carry step-by-step guides on how to make a simple fertiliser bomb.

For those reasons, fertiliser bombs have become preferred weapon for terrorists as diverse as the IRA and al- Qa'ida. It has been produced industrially since the 1930s and is commonly used in agriculture as a nitrogen fertiliser, in solid-fuel rocket propellants, in pyrotechnics and in the production of laughing gas (nitrous oxide).

The full power of the explosive was not discovered until the end of First World War when Fritz Haber won a Nobel prize in 1918 for inventing the ammonia synthesis process. Throughout the war, ammonia synthesis plants were built and used in Germany to supply the country with explosives.

Bali was the most devastating of the attacks in which the substance has recently been used - 202 people, including 26 Britons, died. The truck bomb detonated by Timothy McVeigh in Oklahoma killed 168.

One exception to the rule of it being the explosive preferred by terrorists was the Madrid bombings this month, in which 191 people were killed. In those attacks, dynamite was used.