Julian Assange, who denies allegations he assaulted two women in Sweden, faces arrest if he leaves the embassy.
A WikiLeaks spokesman said the policing costs were “embarrassing”.
Deputy PM Nick Clegg said Julian Assange should go to Sweden and “face justice”.
Between June 2012 and October 2014, direct policing costs were £7.3 million ($11 million), with £1.8 million ($2.8 million) spent on overtime, police said.
Scotland Yard confirmed the cost of the operation to UK taxpayers in the first 28 months, until October 31, 2014, had reached £9 million ($13.5 million).
According to British police, the costs were covered by the budget for diplomatic protection, which provides policing for embassies in the UK.
The cost of a further three months policing is now expected to have taken the total bill to about £10 million ($15 million).
“It is embarrassing to see the UK government spending more on surveillance and detaining an uncharged political refugee than on its investigation into the Iraq war, which killed hundreds of thousands,” WikiLeaks spokesman Kristinn Hrafnsson said.
Julian Assange has attacked Sweden, saying the country had “imported Guantanamo’s most shameful legal practice – indefinite detention without charge”.
In August 2014, Julian Assange indicated he would “soon” leave the embassy, where he has now been for more than 950 days, but he remains inside.
Swedish authorities want to question Julian Assange over allegations that he assaulted two women while he was in Stockholm to give a lecture in 2010.
A Swedish appeal court upheld an arrest warrant against Julian Assange in November 2014.
UK courts have repeatedly ruled that he should be extradited to Sweden to face questioning.
But Julian Assange fears he could be extradited to the US to face charges over the release of top-secret documents by WikiLeaks.
He entered Ecuador’s embassy in London after the UK’s Supreme Court dismissed his bid to reopen his appeal against extradition. He was then granted asylum by Ecuador in August 2012.
Julian Assange has been warned he will be arrested if he leaves the embassy, prompting the 24-hour guard by Metropolitan Police officers.
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