A British businessman who made millions of dollars from selling fake bomb detectors to overseas governments is facing a possible jail sentence after being convicted of fraud.

James McCormick, 56, manufactured and sold the hand-held "ADE 651" devices to countries at serious risk from bombings such as Iraq, claiming they could detect explosives, drugs and other substances.

But in fact they were based on a golf-ball finder, had no working components, lacked any basis in science and did not work in accordance with the known laws of physics, London's Old Bailey court heard.

"James McCormick is a conman," Detective Superintendent Nigel Rock of Avon and Somerset Police said outside court.

"That device has been used and is still being used on checkpoints.

"People using that device believe it works. It does not."

Britain's Press Association quoted prosecutors as saying the detectors were based on a novelty machine for finding golf balls which could be bought in the United States for under $20.

Marketing material claimed items could be detected up to one kilometre underground, at up to five kilometres from the air, and 30 metres under water.

During his trial, McCormick said he had sold his detectors to police in Kenya, the prison service in Hong Kong, the army in Egypt and border control in Thailand.

They were also sold in Niger and Georgia.

Between 2008 and 2010, Iraq bought 6,000 devices at a cost in excess of $US40 million.

"A large proportion of these were countries where there was and still remains a real risk of terrorism and criminality," Superintendent Rock said.

"The devices were used at numerous checkpoints within Iraq during this period.

"It is clear that both civilians and armed forces personnel were put at significant risk in relying upon this equipment."

McCormick said he had had no negative reports from customers.

He will be sentenced next month.

Reuters/ABC