Stephen Fry and Simon Cowell are showing their support for missing children by joining a charity Twitter campaign.

The pair 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.

Kate McCann, mother of missing Madeleine joined the QI host and X Factor supremo, said the campaign “harnesses social media for good”.

Missing People is tweeting a different appeal for a missing child every 30 minutes for 24 hours and encourage their followers to retweet as many as possible.

Around 140,000 children go missing in the UK every year, the charity estimated, and patron Stephen urged users to help “reunite families going through the worst experience imaginable”.

“It is a very simple concept and there is simply no excuse not to get involved,” he said.

“By retweeting these appeals, we can all help extend the platform that might bring those children home.

Circulating the details of these children through the social media site is such a simple, yet incredibly effective way to give the appeals as much coverage as possible.

“I was so impressed by how the world of Twitter responded last year – an incredible 58,000 retweets in one day. The result? Two missing children were found. Invaluable.”

Mrs McCann, writing in the Sun, said: “Every pair of eyes and ears makes a big difference to the search.

“You might be that someone who recognises one of the faces seen on Twitter as you pass them by in the street, or the supermarket, or the train station.”

The charity is using the Twitter handle @missingpeople and the hashtag TheBigTweet until midnight tonight.