Celebrities including Stephen Fry and Simon Cowell have joined a charity Twitter campaign to help find missing children.

The QI host and X Factor supremo reached out to their combined total of nearly 22 million followers in support of Missing People’s Big Tweet, on International Missing Children’s Day.

They were joined by Madeleine McCann’s mother Kate, who said the campaign “harnesses social media for good”.

Kate McCann will also lead a 500-mile (805km) bike ride from Edinburgh to London in June to raise £10,000 for a missing persons charity.

Mrs McCann will lead 20 cyclists on a Cycle Challenge starting at Arthur’s Seat, Edinburgh’s landmark hill, on June 13. They will then take in Newcastle, York, Nottingham and Northampton before finishing at the Tower of London on June 17.

Madeleine’s disappearance is one of the most famous missing persons cases of all time.

Her parents recently won a libel payout against Portuguese detective Goncalo Amaral, who had been on trial over claims he made in a book and a documentary that the couple were involved in Madeleine’s disappearance in Praia da Luz.

In a written verdict, a Lisbon court agreed that Mr Amaral should pay Mr and Mrs McCann 250,000 euros (£179,000) each in damages and it banned further sales of his book, The Truth Of The Lie.

British police officers spent eight days searching three areas of land in Praia da Luz last June, close to where Madeleine disappeared, but found no new evidence.

The Metropolitan Police launched their own operation in 2011.

Missing People is launching a fund-raising effort alongside Mrs McCann’s ride. £MissforMay encourages people to give up a luxury for May and donate the money they save.

Its chief executive, Jo Youle, said: “We are hugely grateful to Kate McCann for taking on this incredible challenge to help missing people and their families across the UK.”

:: To donate to Kate McCann’s Cycle Challenge visit www.justgiving.com/KateMcCann/