British police say they will no longer stand guard outside London's Ecuadorian embassy where WikiLeaks founder Julian Assange took refuge in 2012.

The Metropolitan Police Service (MPS) said it had "withdrawn the physical presence of officers from outside the embassy" but would strengthen a "covert plan" to prevent his departure.

"The operation to arrest Julian Assange does however continue and should he leave the embassy the MPS will make every effort to arrest him," it said.

"Whilst no tactics guarantee success in the event of Julian Assange leaving the embassy, the MPS will deploy a number of overt and covert tactics to arrest him."

Swedish prosecutors want to question Mr Assange about a rape claim, which carries a 10-year statute of limitations that expires in 2020.

Mr Assange, who faces arrest if he tries to leave the embassy, denies the allegation and insists the sexual encounter was consensual.

The Foreign Office said the head of the diplomatic service, Simon McDonald, had summoned Ecuadorean ambassador Carlos Abad Ortiz to insist on a resolution to the impasse.

"The UK has been absolutely clear since June 2012 that we have a legal obligation to extradite Assange to Sweden," the ministry statement said.

"That obligation remains today."

The 24-hour guard outside the embassy in central London has cost taxpayers more than 10 million pounds ($20.8 million), the source of much criticism in austerity-hit Britain.

"Like all public services, MPS resources are finite. With so many different criminal, and other, threats to the city it protects, the current deployment of officers is no longer believed proportionate," police said.

"A significant amount of time has passed since Julian Assange entered the embassy, and despite the efforts of many people there is no imminent prospect of a diplomatic or legal resolution to this issue."

The 44-year-old Australian also fears that if he leaves he could eventually face extradition to the United States and a trial over the leak of hundreds of thousands of classified military and diplomatic documents in 2010.

Swedish officials said in August that they hoped to reach a judicial cooperation deal with Ecuador by year's end that would pave the way for prosecutors to question Mr Assange.

AFP