Britain on Wednesday said it was "determined" to extradite Julian Assange to Sweden to face sexual assault claims as Ecuador said it would reveal its decision over the WikiLeaks founder's asylum claim on Thursday.

"The UK has a legal obligation to extradite Mr Assange to Sweden to face questioning over allegations of sexual offences and we remain determined to fulfil this obligation," said a Foreign Office spokesperson.

The 41-year-old Assange, an Australian national, took refuge at the Ecuadorian embassy in London on 19 June to avoid extradition to Sweden, which he says plans to surrender him to US authorities.

Ecuador's Foreign Minister Ricardo Patino slammed the British government during a press conference on Wednesday, revealing that it had sent a letter to Quito claiming police had the legal right to enter the embassy and arrest Assange.

"They could storm our embassy if Ecuador does not hand over Julian Assange," Patino said.

"The position taken by the government of Great Britain is unacceptable, both from the political and the legal point of view," the foreign minister said.

He warned that entering the embassy without authorisation "would be a flagrant violation of the Vienna Convention" on diplomatic relations.

Britain's Foreign Office later released a statement saying it hoped a "mutually acceptable" solution could still be found, but warned it would do all it could to extradite the former hacker.

"We have an obligation to extradite Mr Assange and it is only right that we give Ecuador the full picture," the spokesperson said of the letter.

"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.

"We are still committed to reaching a mutually acceptable solution."

Patino said that Ecuador "has made a decision" and will announce it Thursday at 7am (1200 GMT). Patino had earlier presented a report on the case to Ecuadorian President Rafael Correa.