Madeleine McCann's parents say they are more confident than at any point for the last four-and-a-half years that their daughter will be found.

Kate and Gerry McCann said they have received a major boost from Scotland Yard's review of the case, and have "no doubt" the Portuguese authorities will eventually re-open their investigation.

In a series of interviews on the eve of the fifth anniversary of Madeleine's disappearance, the couple also said their seven-year-old twins Sean and Amelie have pledged to join the search for their big sister when they are older.

Mr McCann, 43, from Glasgow, said: "I'm certainly more confident now than at any point for the last four-and-a-half years that we actually will find Madeleine and who is responsible."

He added: "I think the most important thing is that a lot of the investigation opportunities are in Portugal. I think it's fairly clear the case will have to be re-opened for those to be pursued adequately.

"We weren't expecting a knee-jerk reaction by any means. This is an ongoing dialogue, and I am sure the investigation will get opened again in due course. I have no doubt about that."

The officer leading Scotland Yard's review of the original investigation into Madeleine's disappearance spoke last week of his belief that the mystery can still be solved and said there is evidence she may still be alive.

Detective Chief Inspector Andy Redwood said the Metropolitan Police would like the case to be re-opened, but stressed the decision was for Portugal.

However, Portuguese police responded by saying they had found "no new element" to justify re-launching their inquiry.

Referring to the Portuguese authorities' apparent reluctance to resume their investigation, Mr McCann said: "The only way everyone will be able to move on is for the case to be solved, and that is for Madeleine to be found and the perpetrators brought to justice.

"Then everyone can move on. Until then, it's not going to go away. It can't go away."

Madeleine was nearly four when she vanished from her family's holiday apartment in Praia da Luz in the Algarve on May 3, 2007, as her parents dined with friends at a tapas restaurant nearby.

The official Portuguese investigation into the case was formally shelved in July 2008, although private detectives employed by the McCanns continued the search.

Scotland Yard's review of the case, called Operation Grange, was launched last May after a request from Home Secretary Theresa May supported by Prime Minister David Cameron.

The Met team has already sifted through 40,000 pieces of information and identified 195 potential leads.

Mr McCann said the Scotland Yard review was "a huge burden off our shoulders" and stressed that the British detectives have uncovered new information that could lead to Madeleine.

"For them to have already identified 195 investigative opportunities – and they aren't halfway through the material yet – does give us a lot of hope," he said.

The couple, from Rothley, Leicestershire, said Sean and Amelie were aware of today's sad anniversary but were "absolutely fine" despite the disappearance of their older sister when they were two.

Mr McCann said: "It's very matter of fact for them, because I suppose they've grown up with Madeleine missing. It's part of their life."

His wife, 44, spoke of wanting to protect the twins, recalling that Sean said to her about 18 months ago: "Shall I come and work for you and find Madeleine when we get older?"

She said: "I still go through that pain, the sadness, anxiety, frustration and anger. It's an awful thing to go through.

"I know how much it's devastated us, and I really don't want Sean and Amelie to have to suffer like that.

"I don't really want them to have the burden of this, of having to keep looking and looking and looking, and not being able to stop. So we need to find her now."

The couple said they were pleased with the new age-progression picture of how Madeleine might look now, approaching her ninth birthday on May 12, which was released by Scotland Yard last week.

They said Mrs McCann's book about Madeleine's disappearance had sold nearly 400,000 copies and raised up to £1 million for the fund to find their daughter.