Under international law, security forces across the world are not allowed to enter an embassy without the express permission of the ambassador – even though the embassy remains the territory of the host nation. The 1961 Vienna Convention on Diplomatic Relations codified the "rule of inviolability", which all nations observe because their own diplomatic missions are otherwise at risk elsewhere.

However, the Foreign Office told Ecuador that it had the power to revoke the embassy's diplomatic status under the Diplomatic and Consular Premises Act 1987. This act was passed by Parliament in the wake of the Libyan embassy crisis three years before, when PC Yvonne Fletcher was shot dead from inside the embassy.

Such a step might set a dangerous precedent by encouraging other governments to justify entering embassies to arrest dissidents seeking diplomatic asylum.

If Ecuador challenged a revocation, ministers would have to argue at the high court that the mission, by harbouring Assange, had itself fallen foul of international law. The government used the power in 1988 to deal with squatters in the Cambodian embassy.

The 1961 convention stresses that missions must respect local laws and not interfere in the host nation's internal affairs. The Metropolitan police says it has the power and right to arrest Assange for breach of bail if he steps outside the embassy.They have also delivered a letter to the embassy demanding that Assange surrender himself.

A legal source at the Ecuador embassy claimed yesterday that the 1987 Act was established to stop a major threat such as terrorism or a nuclear threat.

"The law was established after the shooting of a police officer and would be invoked if the public are under threat. In this case, no one could argue that Assange's presence in this embassy is a threat to the British public," the source said.