Britain’s decision to invoke an obscure domestic law to threaten to strip the Ecuadorian embassy in London of its diplomatic status over the Julian Assange asylum row has caused surprise and is seen as setting a dangerous precedent.

A former British Ambassador to Moscow told the BBC that it appeared inconsistent with the international law on diplomatic relations and could make Britain’s own missions abroad insecure.

The Diplomatic and Consular Premises Act 1987 allows the Government to revoke the diplomatic status of an embassy under certain circumstances — this potentially paves the way for the police to enter the Ecuadorian mission in London to arrest Mr. Assange for breaching his bail conditions.

The Act was introduced after a British policewoman was killed after being shot by someone from inside the Libyan Embassy in London in 1984. It has never been invoked before and many even within the British foreign office are not aware of its existence.

In a letter to the Ecuadorian Government on Wednesday in a bid to dissuade it from granting asylum to Mr. Assange, the British Foreign Office warned that it had legal basis under the Diplomatic and Consular Premises Act “to take actions in order to arrest Mr. Assange in the current premises of the embassy”.

“We very much hope not to get this point [revoking diplomatic status], but if you cannot resolve the issue of Mr. Assange’s presence on your premises, this route is open to us,” it said.

Ecuador’s Foreign Minister Ricardo Patino hit back describing it as “a blatant” disregard for international law and an “explicit type of blackmail”. He said if the threat was carried out it would be interpreted by Ecuador as “an unacceptable, unfriendly and hostile act and as an attempt against our sovereignty which would require us to respond with greater diplomatic force”.

On Thursday, the British Foreign Office sought to play down the threat saying it was committed to finding a negotiated settlement.

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