Ammonium nitrate was found by police at a storage unit in Hanwell

Fertilisers containing ammonium nitrate are widely used in agriculture and are considered "essential" to farmers.

It's very easy to make a bomb with it, provided you know what you're doing Garth Whitty

A purer form of the substance is also used in mining and, when mixed with other substances, including fuel oil, can create a powerful blast.

Ammonium nitrate is used, for the same result, by the British Army as well as armies all over the world.

It was also used in several terrorist bombings in Northern Ireland, as well as in the Bali bombing and the Oklahoma bombing.

'Effective system'

Explosives experts say that to make a bomb from ammonium nitrate fertiliser is relatively straightforward.

Garth Whitty, a former weapons inspector and now head of the Homeland Security department of the Royal United Services Institute, said ammonium nitrate was not effective in isolation.

"But it's very easy to make a bomb with it, provided you know what you're doing", he told BBC News Online.

He said it was equally easy to get hold of the substance and said ammonium nitrate fertilisers could be purified to make them even more effective.

Sooner or later, if people want to get hold of anything then they will be able to eventually Garth Whitty

The main reason people want to get hold of ammonium nitrate is for use in agriculture and to that end, says Mr Whitty, an "effective system" is in place for suppliers to inform the authorities of any irrregular and sudden inquiries about buying large amounts.

"It can, of course, be stolen or acquired by more unscrupulous methods", he added.

He said that, as well as fuel oil, commercial explosive boosters - which are harder to get hold of - were needed to create an ammonium nitrate bomb.

But that would not deter would-be terrorists, he added.

"Sooner or later, if people want to get hold of anything then they will be able to eventually."

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 got a high nitrogen content and if this was used on gardens, it would probably do more damage than good.

"This is reinforced by the fact that it's usually sold in half-tonne quantities, so the size alone would prevent it from being readily available.

"As a man on the street, it would be quite difficult to obtain unless you could steal it in some way. It's not normally sold over the counter.

"It's a fertiliser used by farmers on an industrial scale. But as with many chemicals, it can be manipulated and used for purposes which it was not intended for."