A British surveillance firm was behind the bugging of the London embassy building, in which WikiLeaks founder Julian Assange is sheltering, the Ecuadorean Foreign Minister has claimed.

Ricardo Patino presented the results of tests carried out on what he said was a device found in the office of his country's ambassador to Britain Ana Alban during routine checks in June. He asked for the cooperation of the British government in finding out who is behind the alleged planting of the device.

The hidden microphone could be used to listen to Ms Alban's conversations, Mr Patino said, holding up a picture of the purported bug at a press conference in Quito. Julian Assange lives and works in a different room in the embassy.

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He told reporters: "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."

But the Surveillance Group was quick to deny installing the device. Its chief executive Timothy Young said: "We have this morning heard an accusation the source of which is apparently Ricardo Patino, the Ecuadorian Foreign Minister suggesting that we have bugged the Ecuadorian Embassy.

"This is completely untrue. 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 Ecuadorian Government and our first notification about this incident was via the press this morning. This is a wholly untrue assertion," he told ITV News.

Yesterday, Mr Patino said he would demand "explanations" announcing that a listening device was found during last month's trip to London to visit WikiLeaks founder Julian Assange.

"We are infiltrated everywhere," Mr Patino told reporters at a press conference. He added that he kept the news quiet until then because he "didn't want the theme of [his] visit to London to be confused with this matter." During the trip, Mr Patino also met British Foreign Minister William Hague to discuss Mr Assange's future.

Mr Patino said: "Furthermore, we first wanted to ascertain with precision what could be the origin of this interception device in the office of our ambassador. We are sorry to say so, but this is another instance of a loss of ethics at the international level in relations between governments."

A message posted yesterday on the official WikiLeaks Twitter read: "Sieging/bugging of Ecuador's London embassy and the blockading of Morales jet shows that imperial arrogance is the gift that keeps on giving."

Video: WikiLeaks documentary