"The attorney general's office has decided to reopen the inquiry into the disappearance of Madeleine McCann following a request by police due to new elements," it said in a statement.
The Correio da Manha newspaper had reported that the authorities were considering relaunching the investigation, saying a team was sent to review the case last year with a focus on the possibility of a kidnapping by an organised paedophile network.
Maddie vanished from the holiday apartment where her parents were staying in the resort of Praia da Luz just days before her fourth birthday in May 2007, while they were dining at a nearby tapas bar.
The discovery last week of a young blonde girl in a Roma camp in Greece has resurrected the issue of child kidnapping in Europe once again, and had "given hope" to Madeleine's parents, their spokesman said.
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British detectives investigating the Maddie case last week published two new electronic images of a man they want to contact in a major new appeal also shown in Germany and the Netherlands.
Police also want to identify two other men, probably with short blond hair and possibly speaking German or Dutch, who had also been seen repeatedly near the apartment.
The British detectives said they had received 2,400 calls and emails since the television appeal, while several hundred calls were also received by the German and Dutch shows.
Detective chief inspector Andy Redwood, who is leading the British investigation, said witnesses had spotted a man carrying a young child fitting Madeleine's description towards the port area of Praia da Luz.