Julian Assange remains holed up in the Ecuadorian embassy in London despite being granted political asylum by the South American country while fighting extradition from the UK.
Britain and Ecuador are locked in a diplomatic stand-off over the fate of the WikiLeaks founder, who took refuge in the embassy eight weeks ago after facing extradition to Sweden on allegations of sexual assault.
Mr Assange denies the claims and fears being sent to the United States where he could face the death penalty for publishing secret US diplomatic and military cables.
He described Ecuador's granting him asylum on Thursday as a "significant and historic victory".
However, Foreign Secretary William Hague made it clear that Mr Assange would not be allowed safe passage out of the country, and that diplomatic immunity should not be used to harbour alleged criminals.
Mr Hague said it was a "matter of regret" that the Ecuadorian government granted the WikiLeaks founder political asylum but warned that it "does not change the fundamentals" of the case.
The case could go on for some "considerable" time, Mr Hague said, adding: "We will not allow Mr Assange safe passage out of the United Kingdom, nor is there any legal basis for us to do so."
Britain has warned that it could lift the embassy's diplomatic status to fulfil a legal obligation to extradite Mr Assange by using the Diplomatic and Consular Premises Act 1987.
Ecuadorian ministers responded accusing the UK of threatening to attack the embassy to seize Mr Assange.
Under international law, diplomatic posts are considered the territory of the foreign nation.
But Mr Hague said: "There is no threat here to storm an embassy. We are talking about an Act of Parliament in this country which stresses that it must be used in full conformity with international law."
The Swedish foreign ministry said it has summoned Ecuador's ambassador over the Latin American country's "unacceptable" decision to grant asylum.
News that Mr Assange had been granted asylum was greeted with cheering by protesters kept behind railings across the road from the embassy in Knightsbridge.
The Union of South American Nations (UNASUR) is meeting in Ecuador on Sunday to discuss the situation at the embassy.