By Dominic Evans and Saif Hameed

BEIRUT/BAGHDAD (Reuters) - At a checkpoint in central Beirut, a guard checks a small truck for explosives. He is manning the last security barrier before Lebanon's parliament building 100 meters away, and relying on a bomb detector that experts say is useless.

Holding the device, a swiveling telescopic antenna mounted on a black plastic handgrip, the plain-clothes guard walks by the side of the truck. It does not respond, and the truck is allowed to pass.

At the nearby marina where millionaires' yachts are moored by the glistening Mediterranean Sea, and at entrances to the underground parking of an upmarket shopping mall, the same bomb detectors are used.

They have been a familiar sight at checkpoints across the Middle East for about a decade, acquired for thousands of dollars apiece by authorities desperate to contain deadly waves of bomb attacks.

But the devices - which have even been sold to U.N. peacekeepers - have been condemned by forensic specialists as a dangerous waste of money, based on bogus science.

Marketed under names such as ADE651, GT200 and Alpha, they are supposed to respond to the presence of explosives, causing their metal antenna to swivel on a hinge toward the material.

Britain imposed export bans on ADE651 and GT200 detectors in 2010, warning they were fake, and the British businessmen who made millions of pounds by manufacturing and selling them around the world were subsequently sentenced to jail.

Yet is was only this month that Iraqi Prime Minister Haider al-Abadi - whose country bought hundreds of ADE651 devices eight years ago - ordered his security services to stop using them, after a huge truck bomb killed 292 people in Baghdad.

And devices of a similar design were seen by Reuters correspondents being used at checkpoints in countries including Lebanon, Syria and Egypt in recent weeks or months.

Scientist Dennis McAuley said he examined a device of a similar design to the ADE651 and GT200 when he worked at Northern Ireland's Forensic Science Laboratory. He took it apart to see how it worked.

"There is no scientific basis to it. It's a complete fraud," he told Reuters. "If authorities are putting any reliance on this to detect explosives, it's ludicrous. It's unbelievable they are still using this."

'OBJECTS WITH NO VALUE'

In Egypt, a soldier was seen using one of the wand-like devices this month at a checkpoint in Ras Sidr, checking cars waiting to pass through a tunnel in Sinai. The equipment was used in conjunction with thorough searches of the vehicles.

Egyptian military spokesman Brigadier General Mohamed Samir said any equipment bought by Egypt "is subject to specific standards and is tested before the contracts are signed".

In the Syrian capital Damascus, frequently targeted by rebel and jihadist bombings during Syria's five-year-old conflict, guards outside a hotel and a government complex were seen carrying similar-looking devices in April.

In Beirut, where twin suicide bombings killed 44 people in November, officials declined to comment about the detectors, although guards in the city who were using them this week said they were effective. One said he discovered a concealed package in a car, but did not say whether it contained explosives.

While specific devices seen by Reuters correspondents in Lebanon, Syria and Egypt could not be individually identified, they were of a similar design to the ADE651 and GT200 detectors which Britain imposed export bans on.

Dan Kaszeta, managing director of London-based security consultancy Strongpoint Security, and a graduate of the U.S. military's Explosives Ordnance Disposal school, said no device could work based on the concept of an aerial swiveling in response to traces of explosives it detects.

"Given the state of current technology there is nothing that is in hand-held use that remotely detects explosives with any degree of accuracy or specificity. It just does not exist."

McAuley, the forensic scientist, also said any device based on the swiveling detecting antenna principle - which he compared to rods used for water divining - was fraudulent.

Some customers who bought such detectors never used them. Several years ago, peacekeepers with the United Nations' UNIFIL mission in southern Lebanon purchased some ADE651s but quickly found they were a waste of money.

"We had purchased four of these devices and none of them were working. They were objects with no value," said UNIFIL spokesman Andrea Tenenti.

The British businessman whose company made and sold ADE651 devices, Jim McCormick, was jailed in 2013, three years after Britain banned export of the devices to Iraq or Afghanistan where its soldiers were deployed.

The judge sentencing him said his device was modeled on "failed American manufactured golf ball detectors" and cost less than $50 to make. McCormick sold 7,000 of the devices for $2,500-$30,000 each, with one invoice showing sales worth $38 million to Iraq over a period of three years, the judge said.

The judge added McCormick's fraudulent conduct helped create a false sense of security and probably contributed to deaths of many innocent people.

The businessman behind the GT200 devices, Gary Bolton, was convicted of fraud and sentenced to jail in 2013.

Mexico bought hundreds of the devices. After Bolton's conviction, a Mexican interior ministry official said the devices were no longer in use.

'IT SERVES AS SCARECROW'

In Iraq, even after Abadi's decree, the ADE651 detectors were still in evidence in Salahuddin and Diyala provinces, north of Baghdad.

One police major in Salahuddin said his force had not received written orders to stop using it. Another, in Diyala, said his men had tested the device themselves by stashing a handgun and a grenade one of their vehicles.

The device did not detect them.

Police captain Raad Shallal, manning a checkpoint near the town of Khalis in Diyala province said he knew the detector was useless.

"It serves as a scarecrow, more than a real bomb detector," he added, standing close to a colleague who was checking vehicles with one of the devices.

That theory, that they might deter bombers even if they cannot detect bombs, was lampooned on Iraqi television by satirist Ahmed al-Basheer.

"So it's a scarecrow," he said. "This is the right thing to do, use a device that the entire globe knows is not working in order to scare terrorists who live on the same globe we're on."

Basheer's program broadcast footage from several politicians and officials defending the equipment including Nuri al-Maliki, the former prime minister whose government ordered the devices. He said the first batches worked but subsequent fake deliveries did not.

When ordering the withdrawal of the detectors on July 3, after the truck bombing, Prime Minister Abadi also announced a reopening of an investigation into the "corrupt contracts" involved in the purchase of the devices.

Iraq's judiciary spokesman, Judge Abdul Sattar al-Bayraqdar, said an Iraqi general and several officers were currently in jail, having been convicted of multiple accounts of corruption related to the import of the detectors.

(Additional reporting by Ahmed Rasheed in Baghdad, Asma Alsharif in Cairo and Alizeh Kohari in Mexico City; Editing by Pravin Char)