The cost of the police review of the Madeleine McCann investigation is set to reach nearly £2million.

Scotland Yard’s homicide squad is examining the case of the missing three-year-old after the Prime Minister acted on the request of Madeleine’s parents.

The bill for 30 detectives, translation and expenses comes after David Cameron called in Scotland Yard last May. It is being paid by the Home Office.

A spokesman for Kate and Gerry McCann said: “They have always been very appreciative of the time and resources that the British police and Home Office have committed to the search for Madeleine and they are grateful that the review is ongoing.”

Met detectives have made at least four trips to Portugal and Spain to meet police and private investigators who were engaged in the original investigation.

Labour peer Lord Harris has said the case raises “very big questions”, adding: “There is clearly an issue about the resources being used.”

The cost is disclosed in a document to the London Mayor’s Office for Policing and Crime.

Sources say the Met has already sent the Home Office a bill for £800,000 but the figure is expected to reach £1.9million by the end of the financial year next month.

By comparison, the first three months of the Leveson inquiry into press standards has cost £855,300, figures show.

Operation Weeting, the Metropolitan police investigation into phone hacking, has so far cost just over £1.8m, or £200,000 a month.

The police investigation into the Securitas robbery - Britain’s biggest cash heist – was estimated at £14million.

The gang of five were all convicted after snatching Colin Dixon, his wife Lynn, and their young child at gunpoint to gain entry to the building at Tonbridge, Kent, in February 2006.

Madeleine disappeared from her parents’ holiday apartment in Praia da Luz on the Algarve in Portugal on May 3, 2007.

She vanished days before her fourth birthday as her parents dined with friends yards away. Since then there have been hundreds of “sightings” of Madeleine around the world but none confirmed.

The official police inquiry into her disappearance was shelved in July 2008 but private detectives employed by the McCanns continued the search.

The couple have privately spent £2.5million they have raised on the search.

Private investigators in Spain have handed the Met police team 30 boxes of evidence which they claimed contained up to eight “important new leads”.