Activists on Monday urged Britain to hand over details of the multi-million-dollar sale to Thailand of fake bomb detectors that led to the detention of scores of innocent people.

British fraudsters Gary Bolton and James McCormick were jailed in 2013 for making millions selling the GT200 and similar devices billed as "magic wands" able to detect tiny particles of explosives or drugs from hundreds of metres away.

The GT200 was in fact a useless home-made plastic box with a radio antenna -- made for a few dollars but sold for between $3,300-$13,000 per unit to governments including Thailand, Mexico and Iraq.

On Monday Jatuporn Prompan, the leader of Thailand's pro-democracy 'Red Shirt' street movement, handed a letter to the British embassy in Bangkok urging it to share details of contracts for the device.

British authorities should reveal "particularly the contracts of broker companies which sold them (GT200) to the Thai government and how much they cost", he said in a YouTube post.

A corruption probe into why the Thai military and several other departments ordered hundreds of the devices has ground to a halt.

Opponents of the junta say investigations into allegations of army graft routinely go nowhere, with courts unwilling to tarnish the image of the powerful military.

Anupong Paojinda, the current interior minister and a key player in the 2014 coup that restored the military to power, was army chief when many of the dodgy detectors were ordered around a decade ago.

He repeatedly defended the use of the fake detectors even as tests cast serious doubt over its efficacy.

By 2010 the detection powers of the GT200 had been debunked.

That year Anupong told reporters that "we don't have a replacement yet so we continue to use it".

Last week Prime Minister Prayut Chan-O-Cha, a senior general during Anupong's tenure as army chief, said the device was "useful once... but when they were proven to be useless they were not used anymore".

Rights groups say hundreds of people were detained -- some for several weeks -- in Thailand's restive south after erroneous findings of traces of explosives by security officers using the GT200.

"Regardless of court rulings in the UK and overwhelming scientific evidence, Thai military leadership still defend the use of GT200," said Sunai Phasuk of Human Rights Watch Asia.

He said the first sales took place in 2005, ultimately amounting to orders of nearly 1,400 GT200s worth $32 million across 15 separate Thai agencies.

Sunai called for a probe into possible loss of life and the "large numbers of wrongful arrests" in Thailand's conflict-racked south due to the use of the bogus equipment.

The powerful military refuses to concede it was duped over the acquisition, or apologise to those held in what rights groups call a flagrant miscarriage of justice.