British police have confirmed they will no longer stand guard outside London's Ecuadorian embassy where WikiLeaks founder Julian Assange took refuge in 2012, but will strengthen a "covert plan" to prevent his departure.
Britain's Foreign Office confirmed later on Monday that it had summoned the Ecuadorian ambassador to "register once again our deep frustration at the protracted delay" in extraditing Assange to Sweden to face questions over a rape allegation.
The Metropolitan Police Service (MPS) said on Monday that it had "today ... withdrawn the physical presence of officers from outside the embassy.
"The operation to arrest Julian Assange does however continue and should he leave the embassy the MPS will make every effort to arrest him.
"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," police said in a statement.
Swedish prosecutors want to question Assange about a rape claim, which carries a 10-year statute of limitations that expires in 2020.
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 on Monday that the head of the diplomatic service, Simon McDonald, had summoned Ecuadorian Ambassador Carlos Abad Ortiz to insist on a resolution to the impasse.
The 24-hour guard outside the embassy in central London has cost British taxpayers more than STG10 million ($A20.92 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 on Monday.
"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," they added.
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 Assange.
Britain made a "formal protest" to Ecuador over Assange in August through its ambassador in Quito.
