The cost of the Met's review of the Madeleine McCann investigation is set to reach nearly £2 million a year after it was launched, the Evening Standard reveals today.

The bill for 30 detectives, translation and travel expenses has soared since David Cameron called in Scotland Yard last May.

Detectives from the homicide squad were asked to examine the case of the missing three-year-old after the Prime Minister acted on the request of Madeleine's parents. 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.

When the review was announced it sparked controversy over the use of public funds. 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 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 £1.9 million by the end of the financial year next month.

The bill includes the costs of the detectives' salaries, translation and interpretation fees and travel expenses. 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.

Scotland Yard says the trips to Portugal, and now Spain, are part of "laying the groundwork" for future co-operation between the police forces.

Officers are examining all the

evidence connected to the case, including material gathered by private investigators.

Detectives have spent months reading a huge file of case material that had to be translated from Portuguese to English at a considerable cost. In December detectives met Spanish colleagues in Barcelona to check on reports that the toddler had been abducted and smuggled across the border

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

However, sources say the inquiry could take years to complete and they have played down hopes of a major breakthrough in the review so far.

The cost of the inquiry compares with the £80 million spent on the policing operations to tackle and investigate the summer riots.