The UK has issued a "threat" to enter the Ecuadorian embassy in London to arrest Julian Assange, Ecuador's foreign minister has said.
Ricardo Patino also said a decision on the Wikileaks founder's asylum request would be made public on Thursday.
Mr Assange took refuge at the embassy in June to avoid extradition to Sweden, where he faces questioning over assault and rape claims, which he denies.
The Foreign Office says the UK has a legal obligation to extradite him.
At a news conference in Quito on Wednesday night, Mr Patino said: "Today we received from the United Kingdom an express threat, in writing, that they might storm our Embassy in London if we don't hand over Julian Assange.
"Ecuador rejects in the most emphatic terms the explicit threat of the British official communication."
He said such a threat was "improper of a democratic, civilized and rule abiding country".
"If the measure announced in the British official communication is enacted, it will be interpreted by Ecuador as an unacceptable, unfriendly and hostile act and as an attempt against our sovereignty. It would force us to respond," he said.
"We are not a British colony".
A Foreign Office spokesman said the UK remained "determined" to fulfil its obligation to extradite Mr Assange.
"Throughout this process have 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 to the legal status of diplomatic premises in the UK," the spokesman said.
"We are still committed to reaching a mutually acceptable solution."
The law which Britain is threatening to invoke in the Assange case is the Diplomatic and Consular Premises Act 1987.
It allows the UK to revoke the diplomatic immunity of an embassy on UK soil, which would potentially allow police to enter the building to arrest Mr Assange.
The BBC's deputy political editor James Landale says the British government has been in long negotiations with Ecuador over the issue and has reminded it of the act.
The UK has been frustrated at the lack of a decision but is not about to raid the embassy, he says.
Even if Mr Assange is granted asylum, he will have to cross British territory and could be arrested, our correspondent adds.
On Monday, Ecuador's President Rafael Correa said a decision would be made this week after he held a meeting with his advisers.
Mr Patino told reporters the decision had been made and an announcement would issued on Thursday morning, at 07:00 Ecuadorian time (13:00 BST).
Mr Assange's Wikileaks website published a mass of leaked diplomatic cables that embarrassed several governments, particularly the US in 2010.
The 41-year-old Australian says he fears that if he is extradited to Sweden, he may be sent later to the US and could face espionage charges.