The materials needed to turn ammonium nitrate into a deadly explosive are readily available in the UK.

The chemical was the major ingredient in the biggest of the bombs used by Islamic terrorists in Bali which killed 202 people in 2002.

Normally used as fertiliser, ammonium nitrate can become a powerful explosive when combined with substances including fuel oil.

This mixture was used to make the bomb which killed 168 people in Oklahoma City, USA, in 1995. Half a tonne would be enough for an explosion on that scale.

It was also used widely by the IRA at the height of the Troubles.

Arms expert Mike Yardley said ammonium nitrate was the explosive of choice for terrorists and militants.

The former Army officer said: "It's cheap, readily available and relatively easy to turn into an explosive.

"Terrorists and militants use the ammonium nitrate because it's incredibly stable to store.

"The risk of terrorists injuring themselves while handling ammonium nitrate is dramatically reduced compared with other explosives."

Ammonium nitrate is used by farmers and in the mining industry in weapons-grade form, which is much more tightly regulated.

David Heather, head of the Agricultural Industries Confederation's fertiliser section, said: "I'd be very surprised if you could buy this in a garden centre.

"It's used by farmers on an industrial scale.

"But as with many chemicals, it can be manipulated and used for purposes it was not intended for."