Madeleine went missing aged three while on holiday with her parents in Portugal in 2007 and has never been found.
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David Cameron ordered a review of the case in 2011, which has seen Metropolitan Police detectives travel to Portugal to investigate.
The bill for the British police investigation into Madeleine's disappearance has now topped £10 million, double the £5 million originally set aside.
But the Home Office last night defended the rising cost, telling The Mirror: “The Government believes it is right that it does all it can to support the search for Madeleine McCann.
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“The Home Office will continue to work closely with the Metropolitan Police to review and control the costs appropriately and to ensure that the investigation has the resources it requires.”
The cost of the investigation emerged after a new Portuguese prosecutor, Ines Sequeira, was brought in and given the job of solving the disappearance.
A source close to investigation said: "[Ms Sequeira] has been appointed as the new prosecutor and the Scotland Yard team has been given renewed hope of being able to start work again in Portugal before the end of the month.
"Ms Sequeira is one of the sharpest and most ambitious prosecutors in Portugal. She is utterly determined to get a result on the Madeleine inquiry."
Metropolitan Police officers are planning to return to the Algarve resort of Praia da Luz to re-interview three suspects and have sent a sent a fifth letter to Portuguese officers requesting access to the investigation, which has thrown up seven suspects.
And Madeleine's parents, Kate and Gerry, have been assured the search will go on for the 'foreseeable future'.
A spokesman for the family said: "Madeleine's parents remain extremely grateful for the work carried out by officers from Operation Grange in the continued search for their daughter."