A businessman has been found guilty of a multimillion-pound fraud involving the sale of fake bomb detectors to Iraq and around the world.

A jury at the Old Bailey found Jim McCormick, 57, from near Taunton, Somerset, guilty on three counts of fraud over a scam that included the sale of £55m of devices based on a novelty golfball finder to Iraq. They were installed at checkpoints in Baghdad through which car bombs and suicide bombers passed, killing hundreds of civilians. Last month they remained in use at checkpoints across the Iraqi capital.

McCormick, who faces up to eight years in jail when he is sentenced next month, also sold the detectors to Niger, Syria, Mexico and other countries including Lebanon where a United Nations agency was a client.

He claimed they could detect explosives at long range, deep underground, through lead-lined rooms and multiple buildings. In fact, the handheld devices were useless. Their antennae, which purported to detect explosives, and in other cases narcotics, were not connected to anything, they had no power source and one of the devices was simply the golfball finder with a different sticker on it.

"Both civilians and armed forces personnel were put at significant risk in relying upon this equipment," said Detective Inspector Ed Heath, who led Avon and Somerset police's three-year investigation. "McCormick showed a complete disregard for the safety of those that used and relied upon the device for their own security and protection. He amassed many millions of pounds through his greed and criminal enterprise."

The court heard the devices had been marketed at international trade fairs that were backed by UK government departments. They were only banned from export to Iraq and Afghanistan a year after whistleblowers had alerted the Department for Business and the House of Commons defence select committee.

One of the devices, which were said to be able to detect explosives. Photograph: SWNS.com

It is now alleged by an Iraqi whistleblower that McCormick paid millions of pounds in bribes to senior Iraqis to secure the deals. Inspector general Aqil al-Turehi of the Iraqi interior ministry told a BBC Newsnight investigation: "This gang of Jim McCormick and the Iraqis working with him killed my people in cold blood."

A former colleague of McCormick told the BBC he saw him set up accounts in false names for 15 Iraqi officials. He said they "don't care if people live or die"; the only thing they care about is "how much am I going to get back – cashback".

General Jihad al-Jabiri, who ran the Baghdad bomb squad, is in jail on corruption charges relating to the contracts. Some Iraqis still believe the detectors work while others are angry they have cost lives.

"We know that a few of them are defective," an official at the ministry of the interior told the Guardian last month. "The other problem is how they are used. It requires the operator to be in the right frame of mind."

"We know that the detectors are useless," another security officer said. "They're fakes. We've seen it on the news."

McCormick's home and business premises in Somerset were raided by police in December 2009, by which time British army officers operating in Baghdad and Basra had reported serious concerns about the effectiveness of the devices.

He had sold about 6,000 detectors to Iraq for as much as £10,000 each while the production cost for the device was as low as £15. McCormick grew rich on the enormous mark-up. He owns a mansion in Bath that was previously owned by the Hollywood actor Nicholas Cage, a holiday home in Cyprus and a Sunseeker yacht as part of assets worth £7m identified by police.

One of McCormick's devices, the ADE 101, was essentially a golfball finder described by its maker as "a great novelty item that you should have fun with". The antenna was "no more a radio antenna than a nine-inch nail", according to one scientist who analysed the device.

McCormick also sold three models of his detector to customers in countries including Libya, Iran, Kenya and Jordan. They were used to protect hotels in Lebanon and Bahrain.

The handheld devices came equipped with an antenna that McCormick claimed pointed to explosives or narcotics depending on how it was "programmed". A card reader device could be loaded with a laminated card – orange to look for explosives, blue for narcotics. Expert witnesses described the kit as "completely ineffectual" and an "affront" to science.

During a two-and-a-half-year police investigation, a University of Cambridge professor had conducted a "full double blind trial" of the devices and found the results were no better than random chance. The device was right three out of 25 times.

"His conclusion was that the operating principles of the ADE 651 [another of McCormick's devices] lack any grounding in science and he could see no way in which the device could work according to the presently known laws of physics," the judge told the jury before they retired to consider their verdict.

The court had heard that McCormick continued to believe the devices worked and that none of his customers had returned the devices as faulty or asked for a refund.

Consignments of the devices were once stopped at the UK border before being exported and officials queried whether export documents were required. The reason none were needed was they could find no functioning electronics inside.