Ecuador has accused the UK of making a "threat" to enter its embassy in London to arrest Wikileaks' Julian Assange.
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.
Ecuador says a decision on his claim for political asylum will come later.
The UK Foreign Office says it can lift the embassy's diplomatic status to fulfil a "legal obligation" to extradite the 41-year-old.
In an official note to the Ecuadoreans it said it was "surprised" by media reports that "Ecuador is about to take a decision and proposes to grant asylum to Mr Assange".
The BBC's Tim Reid, outside the embassy, said Ecuador had accused the UK of "intimidating" behaviour and that it was "surprised and disappointed" by what had gone on.
In June, the UK's Supreme Court dismissed Mr Assange's bid to reopen his appeal against extradition and gave him a two-week grace period before extradition proceedings could start.
It was during that fortnight, while on bail, that he sought refuge.
The Wikileaks website he founded published a mass of leaked diplomatic cables that embarrassed several governments, particularly the US's, in 2010.
Mr Assange says he fears that if extradited to Sweden, he will then be passed on to the American authorities.
At a news conference in Quito on Wednesday, Ecuador's Foreign Minister, Ricardo Patino, said a letter from the UK government had been delivered through a British embassy official.
"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," he said.
"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, civilised and rule-abiding country".
He added: "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.
"We are not a British colony."
A Foreign Office spokesman said the UK remained "determined" to fulfil its obligation to extradite Mr Assange.