The government is threatening to enter the Ecuadorian embassy in London and arrest WikiLeaks founder Julian Assange, risking a rift with the country, according to foreign minister Ricardo Patino.
Ecuador's Patino released details of a letter he said was delivered through a British embassy official in the capital of the South American country, Quito, which claimed there was a "legal base" to "take actions in order to arrest Assange in the current premises of the Embassy."
The decision on whether Ecuador will grant Assange political asylum is expected to be announced on Thursday morning.
The Foreign Office denied they were making threats, saying they had a legal obligation to extradite Assange.
The development came two months after Assange suddenly walked into the embassy in a bid to avoid being extradited to Sweden, where he faces allegations of sexual assault.
Police officers outside the Ecudorian Embassy in central London on Wednesday night
Sir Tony Brenton, the UK's former ambassador to Russia, said the Foreign Office may have "overreached themselves" and could risk breaching international law as well as making life "impossible" for those working in embassies.
"The Government itself 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," he said.
"If the Russians had had the power and simply walked into the embassy and simply arrested someone, we would have been in much more insecurity."
The Foreign Office says the Diplomatic and Consular Premises Act 1987 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.
A foreign office source said they understood that the legal process under which Mr Assange could be arrested at the embassy would take seven days to implement.
But Patino said if Britain were to storm the embassy it would "be interpreted by Ecuador as a hostile and intolerable act and also as an attack on our sovereignty, which would require us to respond with greater diplomatic force."
Patino said the situation affected all American states, so the government of Ecuador shall request the convening of a meeting of ministers from across the region.
He added: "The protection that Ecuador offers Mr Assange is based on universal principles and our respect for human rights and no threats of force or unilateral action towards our country will see us waive these principles."
An Ecuadorian government spokesman said on Wednesday: "We are deeply shocked by the British government's threats against the sovereignty of the Ecuadorian Embassy and their suggestion that they may forcibly enter the embassy.
"Instead of threatening violence against the Ecuadorian Embassy, the British Government should use its energy to find a peaceful resolution to this situation which we are aiming to achieve."
The British government's letter to Ecuador added: "We need to reiterate that we consider the continued use of the diplomatic premises in this way incompatible with the Vienna Convention and unsustainable and we have made clear the serious implications that this has for our diplomatic relations."
But a police source said of whether they would storm the embassy: "It's all ifs, buts and maybes at the moment."
Assange denies the allegations against him, but fears he will be sent to the United States if he goes to Sweden.
An offer to the Swedish authorities by Ecuador for investigators to interview Assange inside the London embassy was rejected.
A Foreign Office spokeswoman denied a threat was being made.
The spokeswoman said: "We have consistently made our position clear in our discussions with the government of Ecuador.
"The UK has a legal obligation to extradite Assange to Sweden to face questioning over allegations of sexual offences and we remain determined to fulfil this obligation.
"We have an obligation to extradite Assange and it is only right that we give Ecuador the full picture.
"Throughout this process we have drawn the Ecuadorians' attention to relevant provisions of our law, whether, for example, the extensive human rights safeguards in our extradition procedures, or the legal status of diplomatic premises in the UK.
"We are still committed to reaching a mutually acceptable solution."
In response to the row a Wikileaks spokesperson called for the resignation of William Hague, saying: "We note with interest that this development coincides with the UK Secretary of State William Hague’s assumption of executive responsibilities during the vacation of the Prime Minister and Deputy Prime Minister.
"Mr Hague’s department, the Foreign and Commonwealth Office, has overseen the negotiations to date with Ecuador in the matter of Mr Assange’s asylum bid.
"If Mr Hague has, as would be expected, approved this decision, WikiLeaks calls for his immediate resignation."