The mother of Madeleine McCann is launching a nationwide campaign to find missing people.
A network of billboards will publicise the cases of individuals whose whereabouts are no longer known.
The campaign will see appeals in major cities including London, Birmingham, Manchester and Glasgow running across rail and underground networks, on high streets and in shopping centres.
The advertisements will include details of individual missing people, creating awareness of the new number to call for sightings - 116 000 - and harnessing the public's help to join the search.
Kate McCann, a new ambassador for the charity Missing People, said: "When your child goes missing you want the whole world to stop and join the search.
"More than 250,000 people are reported missing each year in the UK - worryingly they could fill the Olympic Stadium three times over.
"Thankfully, campaigns like this can play a vital part in the search. In May, two youngsters featured in the charity's 'Big Tweet for Missing Children' were found safely.
"The search for Madeleine continues and I am delighted to lend my support as an ambassador to this practical solution that gives hope to families like mine."
The value of advertising space donated by the outdoor advertising industry is estimated at more than £1 million, and the campaign is estimated to reach more than 10 million adults during the summer months when - thanks to the Olympics - Britain will be at its busiest.
The campaign is organised by the Outdoor Media Centre, the trade association for outdoor media owners, and produced by Grand Visual, a production and creative technology company.