Expert in covert techniques says off-the-shelf plug socket device is unlikely to have been planted by state agency

Surveillance experts have described the bugging device that Ecuador says was hidden behind a plug socket in its London embassy as rudimentary and unlikely to have been the work of the British police or security services.

Photographs of the device were revealed late on Wednesday in Quito by Ecuador's foreign minister, Ricardo Patiño, who claimed it had been installed by a Worcester surveillance and security firm, The Surveillance Group Limited.

The firm's chief executive, Timothy Youngs, said on Thursday that the allegation was "completely untrue".

The WikiLeaks founder Julian Assange lives and works in a different room in the embassy in Knightsbridge where he has been granted diplomatic asylum.

After analysing the images, one expert in covert techniques who has worked for UK law enforcement agencies told the Guardian: "We do not do plug sockets, that's old hat. It's the first place people look. The bug is one you can buy off the shelf. If we do something, most likely we would custom build it. My first thought [is] it would not be a state agency."

He said placement in a plug would make the risk of exposure would too great and would only be used as a last resort if installation time was very short.

Another surveillance expert who claimed to have installed several similar bugging devices for clients said the images showed a "bog-standard" piece of equipment.

"We have been installing things like that for 20 years," said Dave Allam, who owns the Spy Shop in Leeds. "I used to be able to install one without switching the electricity off in three to four minutes. The police and security services use a lot more sophisticated equipment than that. I know because I do a lot of debugging."

British police have been known to use plug-socket bugs in the past. During the 1990s investigation into the murder of Stephen Lawrence, the Metropolitan police installed a video and audio bug in a plug socket beside the television set in the flat of Gary Dobson, a suspect in the case.

Allam agreed with the Ecuadorean foreign minister's claim that the device found in the embassy appeared to operate using a mobile phone sim card or possibly an SD memory card.

"This one seems to have a sim card which is integrated into the circuitry so you can phone it from anywhere in the world at any time and it opens up and listens to the conversation in the target area," Allam said.

He said some had a text facility that sent alerts when people were speaking in the room so that users could call to listen in.

"Everybody uses these," he said. "It ranges from businesses wanting to know what their employees are doing, husbands and wives not knowing what the other is doing, anyone who wants to get ahead of the game."

Holding up a picture of the purported bug at a news conference in Quito, Patiño said the purpose of the hidden microphone was to listen to the conversations of the ambassador, Ana Alban, in her office.

He said: "We have reason to believe that the bugging was carried out by The Surveillance Group Limited, one of the largest private investigation and covert surveillance companies in the United Kingdom."

In response, Youngs said: "The Surveillance Group do not and have never been engaged in any activities of this nature. We have not been contacted by any member of the Ecuadorean government and our first notification about this incident was via the press this morning. This is a wholly untrue assertion."

The Surveillance Group website states that its clients include "police, government agencies and local authorities in relation to drugs, prostitution, gang violence, hate crime and antisocial behaviour". It says its services "combine the practices, skills and experience of special forces, police and commercial surveillance". It declined to comment further.

It remains unclear why Ecuador believes the device was installed by The Surveillance Group, and the embassy declined to elaborate on Patiño's claim. Patiño said Ecuador would ask for the co-operation of Britain's government in investigating the alleged bugging.

A spokesman for the Foreign and Commonwealth Office said the British government had yet to receive an official request from Quito for co-operation, but added that the UK would consider any such appeal in principle.