Farmers are being warned to keep fertiliser locked up to stop it being stolen and turned into bombs by terrorists.

Police fear extremists could use the lack of security at Sussex farms to get their hands on the components to make deadly weapons.

In May 2007 Omar Khyam, Jawad Akbar and Waheed Mahmood, all from Crawley, were jailed for life for conspiring to cause explosions likely to endanger life between January 2003 and March 2004.

They had targeted a nightclub, shopping centre and gas network with a giant bomb after buying 600kg of ammonium nitrate from an agricultural merchants, which they kept at a storage unit in West London.

Sussex Police has issued the stark warning about fertiliser security on its Farmline information service, an advice service for farmers which provides farming news, weather, prices and information.

The message reminds farmers that ammonium nitrate fertiliser has been used by “certain extremist groups” to make bombs.

And it directs farmers to a government website for advice on the secure storage of the potentially-deadly substance.

The Sussex Police warning says: “We understand that in modern farming it is unusual to have such chemicals hanging around in your yards before use, or to order more than is necessary.

“Never the less, for the time it is on your land it is potentially an easily accessible source of this chemical for terrorists.”

Farmer Frank Grantham, of Old Erringham Farm House in Steyning Road, Shoreham, said he numbered his bags of fertiliser so he could instantly tell if one was missing.

He said: “It's a scary thought. If a farmer had some stolen from him and it was found out that it was fertiliser used for untoward purposes, the guilt might kick in a bit.”

Mr Grantham, who has lived and worked on the farm all his life, said he receives text messages from Farmwatch which advises farmers to keep an eye on their fertiliser.

He said: “We will not use it until next spring so it's something you have to keep an eye on.”

A ten point plan for fertiliser security is also listed on the website and gives advice which includes not storing fertiliser where there is public access, not leaving bags of fertiliser in fields overnight, not selling fertiliser unless the buyer is personally known to you to be a bona fide farm user and making sure buyers are aware of the need to follow the website's guidance and recording fertiliser deliveries and usage.

For more information visit the website at www.secureyourfertiliser.gov.uk.