Coral Jones and Mrs McCann, mother of missing Madeleine, unveiled a digital billboard at King's Cross station in London promoting an enhanced Child Rescue Alerts system to inform the public about missing children whose lives are considered at risk.
The scheme will use social media including text messages, email and digital billboards across the UK in addition to traditional broadcast media to issue the alerts when it is launched on Sunday, International Missing Children's Day.
The scheme, the responsibility of the National Crime Agency (NCA) in partnership with the charity Missing People and the company Groupcall Limited, already has 25,000 people signed up. Organisers say the scheme will be able to issue localised and national alerts and has a target of a million to sign up by the end of its first year.
Chris Hedges, manager for Missing and Abducted Children at Ceop, part of the NCA, said: "It enables us to disseminate messages either at a very localised level or nationally quickly and effectively whenever there is a particularly highly endangered child we are trying to locate."