Missing Madeleine McCann could be found alive, Britain's most senior police officer has said.
Sir Paul Stephenson said it is right for Scotland Yard to agree to Home Secretary Theresa May's request for help and said there is "always a chance" that the young girl could be found alive.
His comments come after the decision to involve the Metropolitan Police was described as a "ludicrous" move that would deny other victims of crime the chance of justice while using up valuable police resources.
But the Commissioner told LBC radio it is not unique for the Met to lend its expertise in difficult cases outside of its normal jurisdiction, saying a team were sent to Antigua in the summer of 2008 to help investigate the murders of British honeymooners Ben and Catherine Mullany.
He said: "There is always a chance. When you receive a request, supported by the Prime Minister, from the Home Secretary, you take that very seriously. This is not unique. On balance, I think it was the right thing to do."
But the review, which will be funded by the Home Office, has already sparked fierce criticism amid suggestions the intervention could undermine the independence of the force.
Last week, Lord Harris, a member of the Metropolitan Police Authority (MPA), said: "Whilst no-one doubts the desirability of doing what can sensibly be done to find out what has happened to Madeleine McCann, I can imagine that the senior leadership of the Metropolitan Police are not exactly happy about this.
"It again embroils their officers in a high-profile investigation, where the chances of success are unclear, and which will divert limited investigative resources away from other matters."
Fellow MPA member Jenny Jones told the Mail on Sunday: "It is ludicrous. This could take years and will cost millions. It is very unusual for police to step in like this and it is not an appropriate use of police resources."
Madeleine's parents, Kate and Gerry McCann, have welcomed the move. Their daughter went missing from her family's holiday flat in Praia da Luz in the Algarve on May 3 2007, shortly before her fourth birthday.