Ecuadorian Foreign Minister Ricardo Patino made the charge Wednesday at a news conference in Quito, The Independent reported. Patino told reporters Ecuador has "reason to believe" The Surveillance Group Ltd. installed the device.
Timothy Young, the group's chief executive, denied the accusation.
"This is completely untrue. The Surveillance Group do not and have never been engaged in any activities of this nature," he told ITV News. "We have not been contacted by any member of the Ecuadorian government and our first notification about this incident was via the press this morning. This is a wholly untrue assertion."
WikiLeaks founder Julian Assange has been living in the London embassy for months. He took refuge there to avoid extradition to Sweden, where he is under investigation for alleged sexual assault.
An expert on surveillance who has worked for British government agencies told The Guardian he does not believe the device found in the embassy was placed by one of the British security services.
The device was found in June in an electrical socket in the office of Ambassador Ana Alban, the report said.
"We do not do plug sockets, that's old hat. It's the first place people look," the expert said. "The bug is one you can buy off the shelf. If we do something, most likely we would custom build it."