A hidden microphone has been found inside the Ecuadorean embassy in London, where the WikiLeaks founder Julian Assange is holed up, according to the country's foreign minister.

Ricardo Patiño said the device had been discovered a fortnight ago inside the office of the Ecuadorean ambassador, Ana Alban, while he was in the UK to meet Assange and discuss the whistleblower's plight with the British foreign secretary, William Hague.

"We regret to inform you that in our embassy in London we have found a hidden microphone," Patiño told a news conference in Quito on Tuesday.

"I didn't report this at the time because we didn't want the theme of our visit to London to be confused with this matter," he said.

"Furthermore, we first wanted to ascertain with precision the origin of this interception device in the office of our ambassador."

He described the discovery of the device as "another instance of a loss of ethics at the international level in relations between governments" and said he would reveal more details as to who might have planted the microphone on Wednesday.

The Foreign Office declined to comment immediately on the allegation, while a No 10 spokesman said he did not comment on security issues.

Assange has been living inside the embassy for more than a year to avoid extradition to Sweden to face allegations by two women of sexual assault and rape, which he denies.

He fears that if sent to Sweden he could be extradited from there to the US to face potential charges over the release of thousands of confidential US documents on WikiLeaks.

Ecuador's protection of Assange has strained relations with Britain.

After talking to Assange until four in the morning on 17 June, Patiño met Hague for fruitless negotiations on the affair.

The Foreign Office said "no substantive progress" had been made during the discussions, while Patiño told a press conference that Assange's situation was "totally unjust".

Patiño also said his government and Assange himself were prepared for a long waiting game, with the WikiLeaks founder telling him he was fit enough to spend another five years inside the Knightsbridge embassy.

Staff at the embassy are understood to have found the heavy police presence around their office intimidating. The round-the-clock police guard has so far cost more than £3.3m and has been described by Patiño as "a bit uncomfortable" for embassy employees.

Despite the discomfort and the diplomatic impasse, however, the Ecuadorean foreign minister has been careful to stress that his government had no intention of using espionage sleight-of-hand to whisk Assange off the premises.

"We're not going to have Mr Assange escaping from our embassy; we're not going to come up with some undercover operation to smuggle him out," he told reporters following his June meeting with Hague.

"We want to be open about this, we want to discuss the issue with the UK government … but if they insist on putting the police around our embassy, then OK, we respect that. But it's important that they know that Ecuador isn't going to do anything irregular; we're not going to smuggle Mr Assange out in the boot of a car or through and underground tunnel or something.

"The Ecuadorean government isn't going to go out through the back door, we're going to come out proud through the front door. We're going to ensure that he doesn't get smuggled out in a vehicle; we're going to ensure that he comes out facing his freedom - the freedom of our country. This is what we are hoping for."