Ecuador has accused Britain of threatening to enter its embassy in London to arrest the Wikileaks founder, Julian Assange.
Carl Gardner, a former UK government lawyer, said this would be "uncharted territory".
Speaking to the Today programme, he said it was not clear whether exercising an obscure piece of legislation passed 25 years ago that would allow the British government to change the status of the embassy and go in and arrest Mr Assange would be lawful if used for that purpose.
Former ambassador Sir Tony Brenton told James Naughtie the Foreign Office had "slightly overreached themselves". And he thought it "highly unlikely" that the government would storm the embassy.
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