Metropolitan Police officers have been stationed outside the embassy in Knightsbridge around the clock since June.
The Met said the estimated total cost of policing the embassy between last June and the end of January is £2.9 million, of which £2.3 million is opportunity costs – police officer pay costs that would be incurred in normal duties – and £600,000 overtime.
A spokesman at the Ecuadorian embassy said: "The Ecuadorian government is concerned by the significant cost to the taxpayers of London of policing the embassy at the present time.
"However, we believe this expenditure could be avoided if the UK Government would provide the undertakings that the Ecuadorian government has sought that there will be no onward extradition of Julian Assange to the United States.
"The Home Office has the power to offer such an assurance, but has so far declined to do so."
Mr Assange, who is fighting extradition to Sweden on sex charges, which he denies, amid fears he will be sent to the US, has been granted political asylum by Ecuador.