The video shows computer-generated images of what Madeleine McCann may now look like
Internet users globally are being urged to spread a video message aimed at "pricking the conscience" of people who know what happened to Madeleine McCann.
UK detectives are targeting the film - with new images of how the girl might look now - at Madeleine's possible abductor or people close to them.
Her father Gerry said: "We have to get under the skin of [those] that know."
Madeleine, of Rothley, Leicestershire, was nearly four when she vanished from an Algarve holiday flat on 3 May 2007.
Despite a massive police investigation and huge publicity worldwide, there has been no sign of her since.
The one-minute video message - produced in seven languages - has been launched by Britain's Child Exploitation and Online Protection Centre (Ceop) in an attempt to secure vital information about Madeleine's whereabouts.
Police hope people will spread the new film via blogs, e-mail and social networking sites such as Facebook and Twitter.
The new computer-generated images of how Madeleine might look now include one of her with dark brown hair and tanned skin, in case she has been living in north Africa.
In May this year an image was released by the US National Center for Missing and Exploited Children (NCMEC) of how Madeleine might have looked aged six.
But following concerns that it looked "too American", two fresh pictures have been produced.
In the video, Ceop's Jim Gamble appeals directly to anyone with information about what happened to Madeleine.
He says: "We know that there is someone out there who knows who is involved in her disappearance. They may be keeping this secret out of fear, misplaced loyalty or even love.
Madeleine was days short of her fourth birthday when she disappeared
"Keeping this information secret only increases the anguish of Madeleine's family and friends and increases the risk to other children.
"If you know who is involved and are keeping this secret, remember that it is never too late to do the right thing."
He said the primary aim was "pricking someone's conscience, making them realise there is an opportunity to help" by coming forward with information.
"The people around [the perpetrator]... are being made complicit in this - it is time to turn a negative into a positive," he added.
He said the person they hoped to reach was likely to be a partner, family member, friend or colleague of the person or people involved.
He added Ceop had consulted psychologists about guilt and how the campaign could "open the opportunity for an individual or remind them that they can redeem themselves."
Mr McCann told BBC Breakfast that because the message was targeted at a small number of people, the public's help was needed to spread it "far and wide" and expose a potential abductor and people close to them to it as many times as possible.
Mr McCann and his wife Kate have campaigned vigorously to keep the case in the spotlight.
But Mrs McCann said: "We haven't found Madeleine so we haven't done enough, which is why we're doing this appeal."
She added she felt hopeful that the message would generate some new information.
Mr Gamble said the appeal had come about after the McCanns wrote to him expressing frustration that lines of inquiry were drying up.
The McCanns returned to Portugal in September as part of their campaign
The message is available in English, Arabic, French, German, Italian, Portuguese and Spanish.
It is being supported by police agencies around the world, including Interpol, Europol and forces in Australia, the US, Canada and the United Arab Emirates.
Although Ceop does not work directly with the Portuguese police - who shelved their investigation in July last year - any relevant information received will be passed on to Leicestershire Police, who will share it with detectives in Portugal.
Mr Gamble stressed that investigations involving missing children were never closed, citing the cases of youngsters such as Jaycee Lee Dugard, who turned up in California in August this year after disappearing 18 years earlier.
The McCanns have changed their official findmadeleine.com website to open with the message: "It's never too late to do the right thing."
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