Twelve Britons are on a list of possible suspects in a new criminal inquiry into the case of missing Madeleine McCann, it was revealed today.

Scotland Yard announced it was launching a full-scale investigation into her disappearance and repeated an assertion that she could still be alive. Detectives revealed they had identified a total of 38 people who were “of interest” — including 12 Britons who were in Portugal at the time Madeleine vanished.

The Met said officers were working closely with the Portuguese authorities in an almost unprecedented case involving a British police force probing a crime committed abroad. Home Secretary Theresa May has agreed to fund the new inquiry, which follows a two-year, £5 million review of the evidence.

Madeleine was nearly four when she went missing from her family’s holiday apartment in Praia da Luz on the Algarve in 2007 as her parents dined with friends nearby.

A Portuguese probe was shelved in 2008 but the Yard review was launched in 2011 following the intervention of David Cameron at the request of the McCann family.

Det Ch Insp Andy Redwood, leading the inquiry, said as he launched the investigation today: “The review has given us new thinking, new theories, new evidence and new witnesses.”

He said officers had identified 38 “persons of interest” from four European countries, including Portugal, who will be the subject of the new probe. He said neither the McCann family nor their friends staying with them at the time were among the 38 people they had identified.

Police are believed to have identified hundreds of possible new leads as well as opportunities to develop new forensic evidence. Detectives revealed they have also interviewed a number of people in Britain as witnesses. They are thought to have been holidaymakers and friends of the McCanns who were with them in the resort at the time.

But insiders emphasise that there is no prime suspect in the investigation, which is still in its early days.

DCI Redwood said: “We and the Portuguese authorities remain completely committed to finding out what happened to Madeleine and everything we do is focused on her best interests.

“We continue to believe there’s a possibility Madeleine is alive. It is a positive step in our hunt for her that our understanding of the evidence has enabled us to shift from review to investigation.”

A spokesman for Madeleine’s parents said today: “Kate and Gerry warmly welcome the shift in the Met’s emphasis from review to investigation. It is clearly a big step forward in establishing what happened and, hopefully, towards bringing whoever is responsible for Madeleine’s abduction to justice.”

Detectives have asked the Crown Prosecution Service to submit a formal request to the authorities in Portugal for assistance in the inquiry. It is understood officers have built up a close working relationship with their Portuguese colleagues who will need to carry out inquiries on their behalf.

Yard officers and CPS officials have travelled to Portugal 16 times in the past few years and shared their findings with the authorities there.

A small number of British detectives are expected to be based in Portugal while inquiries unfold.

The Yard has launched a new investigation because Portuguese authorities are unable, under their law, to re-open their inquiry unless there is compelling new evidence. They say the new leads identified by the Met inquiry do not yet justify a fresh investigation.

However, the Yard believes the leads could generate new evidence and even lead to the case being solved.

The new investigation also raises the possibility of a trial in the UK if a suspect who is a British national is identified and arrested.