Martin Alipaz / EPA file
A composite file photo of Wikileaks founder Julian Assange , left, and Ecuador's President Rafael Correa, right.
"We are not a British colony," Ecuadorean Foreign Minister Ricardo Patino said in an angry statement.
"Under British law we can give them a week's notice before entering the premises and the embassy will no longer have diplomatic protection," a Foreign Office spokesman said.
"But that decision has not yet been taken. We are not going to do this overnight. We want to stress that we want a diplomatically agreeable solution."
The decision on Assange's asylum request would be announced on Thursday at 7 a.m. (12 GMT), Reuters reported.
Former computer hacker Assange, who enraged Washington in 2010 when his WikiLeaks website published secret U.S. diplomatic cables, is wanted in Sweden to face trial for rape.
Assange has been taking refuge in the Ecuadorean embassy in London since June 19. The Australian anti-secrecy campaigner says he fears he could be bundled to the United States where his life would be at risk.
President Rafael Correa, who is openly sympathetic to Assange, said via Twitter on Tuesday that a "rumor of asylum for Assange is false. A decision has not yet been made. I am awaiting a report from the Foreign Ministry." Britain's Guardian newspaper had earlier quoted an unnamed Ecuadorean official as saying asylum had been granted for the Australian.
However, granting asylum would offer no legal protection in Britain where police will arrest him once they get a chance.
Assange has no way of leaving his refuge in the Ecuadorean embassy in London without being arrested, even if Quito grants him asylum shortly, lawyers say.
"I can't see the UK backing down and just allowing him safe passage out of the country," said Rebecca Niblock, an extradition specialist at London law firm Kingsley Napley.
"I think the UK will see their obligations under the European extradition system as overriding any diplomatic relations with Ecuador, who haven't really been considering their diplomatic relations with the UK, apparently."
Assange would be protected from arrest if travelling in a diplomatic car, but the embassy is on the first floor of a building that is being watched by police day and night.
The tall red-brick block just behind the Harrods department store also houses the Colombian embassy and private apartments. A police van was parked outside the main entrance on Wednesday and police officers were patrolling the area in pairs.
The property has several gated entrances and a private car park, but the Ecuadorean embassy is not linked internally with any of them, making the front entrance its only point of exit, a security manager at the building told Reuters.
"There is no other exit. He is going to have to come out of the main entrance," said the manager, who asked not to be named. "There is no way to bring a vehicle in because the car park is private and it is not connected in any way to their premises."
He added: "He can climb out of a window, of course, but there are CCTV cameras everywhere."
Even if he somehow managed to get out of the building and into a waiting car unnoticed by police, he would have to leave the vehicle at some point to board a flight out of Britain, offering more opportunities for his arrest.
Other scenarios lawyers are discussing on the Internet include smuggling him out in a diplomatic bag, which would be illegal, or appointing him as an Ecuadorean diplomat to give him immunity. But lawyers and diplomats said neither was realistic.
Even if Assange were willing to try his luck packed in a crate all the way to Quito, a risky plan by any measure, it seems unlikely Ecuador would attempt such a scheme.
Reuters and The Associated Press contributed to this report.
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