Ecuador accused Britain on Wednesday of threatening to storm its London embassy to arrest Julian Assange after the U.K. issued a stern warning to the South American nation ahead of its decision on an asylum bid by the WikiLeaks founder.

Foreign Minister Ricardo Patino said Britain had earlier in the day issued "a written threat that it could assault our embassy" if Assange is not handed over.

Patino also said he would announce on Thursday morning whether Ecuador would grant the request of the secret-spilling former Australian hacker, who took refuge in Ecuador's embassy on June 19 to avoid extradition to Sweden. Assange faces questioning there for alleged sexual misconduct.

As news broke of the warning, police were seen reinforcing Scotland Yard's presence outside the embassy in a tony London neighbourhood near the Harrods department store.

In Quito, about 30 people yelling "England, what part don't you understand? We are sovereign!" protested outside the British Embassy, and briefly trampled a British flag.

In London, a small group of Assange supporters were gathered outside the Ecuadorean embassy late Wednesday, according to live footage broadcast by a citizen journalist on the scene. The embassy was dark, although occasionally the curtains appeared to move.

WikiLeaks founder Julian Assange took refuge in Ecuador's embassy on June 19 to avoid extradition. (Lefteris Pitarakis/Associated Press)

British officials have vowed not to grant Assange safe passage out of their country if Ecuador grants asylum. They say they will arrest him the moment he steps outside the embassy.

But they had not publicly suggested they might strip the embassy of its diplomatic inviolability.

After Patino's brief appearance before reporters, Britain's Foreign Office issued a statement citing a 1987 British law it says permits the revocation of diplomatic status of a building if the foreign power occupying it "ceases to use land for the purposes of its mission or exclusively for the purposes of a consular post."

Under international law, diplomatic posts are considered the territory of the foreign nation.

Asked by The Associated Press about Patino's characterization of Britain's warning, a Foreign Office official said via email that the letter "was not a threat" and was intended to clarify "all aspects of British law that Ecuador should be aware of." The official would not be identified by name, citing policy.

U.K. has ‘obligation to extradite’

Patino said the missive including the veiled threat was delivered to his country's Foreign Ministry in writing and verbally to its ambassador in London on Wednesday. The law cited was Britain's 1987 Diplomatic and Consular Premises Act.

Patino said Ecuador "rejects in the most energetic terms the explicit threat of the official British communication."

The Foreign Office statement did not elaborate on Britain's intentions if Assange were to be granted political asylum by Ecuador, whose president, Rafael Correa, has expressed sympathy for the Wikileaks founder.

"We have an obligation to extradite Mr. Assange and it is only right that we give Ecuador [the] full picture," the statement said, before adding: "We are still committed to reaching a mutually acceptable solution."

Assange, whose publishing via the internet of thousands of sensitive U.S. diplomatic cables and military dispatches has angered U.S. officials, says the charges against him are trumped up.

His supporters say they believe the U.S. has secretly indicted him and would extradite him from Sweden.

Correa has said Assange could face the death penalty in the United States and for that reason he considers the asylum request a question of political persecution.

Analysts in Ecuador expressed doubts that Britain would raid the embassy.

Professor Julio Echeverria of Quito's FLACSO university said Britain "has a long established tradition in Europe of respecting diplomatic missions," which under international law are considered sovereign territory.

A former Ecuadorean ambassador to London, Mauricio Gandara, told The Associated Press "I refuse to believe in this threat because if asylum is granted the British government will not grant safe passage and Mr. Assange could be in the embassy for a long time."