Madeleine McCann has been missing since Thursday

John Buck said he had been assured by the Portuguese authorities that "everything possible" was being done to ensure the girl's safe return.

His comments came after police said they had stepped up their efforts.

The three-year-old, from Rothley, Leicestershire, disappeared from an apartment in Praia da Luz.

Portuguese officers, who have faced criticism of their operation, said they had quizzed 100 people and searched 500 apartments.

Mr Buck said he had met Madeleine's parents Gerry and Kate McCann again o­n Monday.

He said additional British experts had arrived to work with the Portuguese o­n the case.

Mr Buck said: "This is, and must remain, a Portuguese police investigation.

"As you know, the Portuguese police operate under Portuguese law, and Portuguese law puts constraints o­n what they can say publicly and the information they can release."

He added: "I, and I know Kate and Gerry, with whom I've just been speaking for the past hour, are very grateful for their efforts."

'Mistaken identity'

Earlier Portuguese television had reported that a man and a child matching Madeleine's description were seen driving off in a car from the town of Nelas, about 300 miles (483km) north of Praia da Luz.

However, police, who are following up 350 pieces of information, said the sighting was a case of mistaken identity.

Chief Inspector Olegario de Sousa, of the Portuguese police, said: "It was not Madeleine. It was a child who looks like Madeleine, but she is a child who lives in the area."

A sign saying 'Come Home Soon Madeleine' was seen at a vigil

He said officers were maintaining a "high level" of effort to find Madeleine, as well as gathering evidence about her disappearance.

The force has been criticised for not keeping the public informed.

Mr Sousa's press conference, where he listed the action taken by his officers, came after intense pressure from the media.

He told assembled journalists that an area with a radius of 15km (9.3 miles) around the Algarve resort had been searched - along with hundreds of nearby apartments.

Antonio Santana Carlos, the country's ambassador in London, said the case was of "great concern" to Portugal and asked people to trust the police.

BBC correspondent Alison Roberts said the story had been widely covered o­n Portuguese television and radio, and was making the front page of newspapers.

However, she said, some Portuguese journalists have asked whether so many resources had been poured into the investigation because of Madeleine's nationality, and whether the same effort would have been made for a Portuguese child - an accusation quickly denied by police.

Footballers including England captain John Terry and Portugal internationals Cristiano Ronaldo and Paulo Ferreira, who both play in England, have appealed for anyone with information to come forward.

A set of pyjamas identical to the o­nes Madeleine was wearing

Meanwhile, in the McCanns' home village of Rothley, residents held a silent vigil to show their support.

Staff at Leicester's three hospitals also joined in prayer for the McCann family.

Madeleine's father is a consultant cardiologist at the city's Glenfield Hospital and her mother is a GP.

The McCann family have confirmed Madeleine was wearing white pyjama bottoms with a small floral design and a short-sleeved pink top with a picture of Winnie the Pooh character Eeyore when she disappeared.

She is believed to have been taken from her bed last Thursday evening while her parents were eating at a nearby restaurant.

They had been regularly checking o­n her and their two-year-old twins, Sean and Amelie, before she went missing.