A protester with the Anonymous' trademarking Guy Fawkes mask (Twitter)

A former UK ambassador to Russia has warned that the Foreign Office risks breaching international law if it decides to overturn the status of the Ecuador Embassy where WikiLeaks founder Julian Assange has sought asylum to avoid extradition to Sweden.

Tony Brenton, who served as UK ambassador in Moscow from 2004-08, said Britain's threat to withdraw diplomatic status for the South American embassy would make life "impossible" for British diplomats overseas.

The deadlock has bought a growing number of protesters, some dressed in "Anonymous" masks, on to the streets around the embassy in Knightsbridge, central London.

"The Foreign Office has slightly overreached themselves here, for both practical and legal reasons," he told BBC Radio 4's Today Programme.

"The government has no interest in creating a situation where it is possible for governments everywhere to arbitrarily cut off diplomatic immunity. It would be very bad."

The Foreign Office threated the Ecuadorian government in a letter that it would "take actions in order to arrest Mr Assange in the current premises of the embassy".

The disputed letter

The letter, delivered by local British diplomats in Quito, makes reference to the Diplomatic and Consular Premises Act 1987 which gives ministers power to scrap recognition of diplomatic premises.

It was introduced following the 11-day 1984 Libyan Embassy siege, when an armed gunman shot and killed WPC Yvonne Fletcher.

The law was designed to prevent terrorism or other breaches of international law within a foreign embassy. Ecuador has not been accused of any such thing.

Britain could be in breach of the Vienna Convention which protects the status of embassies and diplomats if it withdraws diplomatic status.

"The premises of the mission shall be inviolable," reads the convention. "The agents of the receiving state may not enter them, except with the consent of the head of mission.

Under its protocols, Assange cannot be arrested so long as Ecuadorian diplomats protect him inside their embassy. They could, however, invite the police inside to make their arrest.

Even if Ecuador grants asylum to Assange, the Foreign Office pointed out that the UK would refuse safe passage for the WikiLeaks founder.

"Giving Julian Assange asylum will not change anything. Britain has a legal duty to extradite him to Sweden," tweeted the official account of the Foreign Office.

Exit Options

The Vienna Convention says that Assange could try to get to the airport in an Ecuadorian ambassadorial car. But as Carl Gardner points out in his Head of Legal blog: "Assange would have to get into any car somehow before being arrested on the pavement" - closing that option to him.

Another option is that Assange might be smuggled out in a "diplomatic bag", which is also protected by the Vienna Convention.

It has been tried before. In the 1980s Nigerian government agents tried to smuggle Umaru Dikko out of Britain. However, the state can insist on a diplomatic bag being opened or returned and it is hard to predict whether an airline would agree to carry it.

"The diplomatic bag idea really is best left to fiction," says Gardner.

The most realistic possibility is appointing Assange as a "diplomatic courier" with the task of transporting a diplomatic bag. The courier enjoys "personal inviolability and shall not be liable to any form of arrest or detention", under the convention.

However, London has always had the right to reject him as "persona non grata". He could only claim immunity "in the performance of his functions" as a courier.

Ecuador could appoint Assange one of its UN representatives under rule 25 of the UN General Assembly's Rules of Procedure.

Protesters have massed outside the embassy and there is a large police presence. At least three protesters have been arrested.