Catherine joins William as he says sport can save lives from suicide

The On Your Side project wants to give people confidence to start the "difficult conversations" with friends, relations, teammates or colleagues who might be struggling with their mental health and might be at risk of suicide.

It is being supported by sport stars including England football captain Harry Kane, retired British middle distance athlete Dame Kelly Holmes, and former England football star Alan Shearer.

Royal sources said that Catherine wanted to attend to "highlight the difference that can be made when people feel able to talk openly about how they are feeling and when someone is there to listen".

"Never underestimate the difference that you can make," said Prince William, stressing that suicide prevention was "not someone else's responsibility". "It belongs to all of us," he said.

Talking of hidden anxieties, he added: "We do not always see what is happening behind closed doors, or understand the battles that may be taking place beneath the surface."

Dame Kelly talked of her own struggles: "We talk about suicide, which is the end for that journey. What we forget is that journey has happened for a long period of time. What we need to avoid is that journey coming to that endpoint."

It was important that sports stars admitted their own vulnerability, said Olympic cyclist Dame Laura Kenny.

She spoke movingly of how a family member had tried to take their life and how she felt "almost ashamed to admit that this could happen in my family because it's not a subject that is easily spoken about".

The cyclist also described the darkness of her feelings after the loss of two babies, through a miscarriage and an ectopic pregnancy, "which ultimately ended up in an emergency situation".

On Your Side aims to use the reach of sport to raise awareness about suicide and to show people the help available in a crisis, with the Football Association, Premier League and Team GB providing support.

The level of suicide is disproportionately higher among men than women - it is the biggest cause of death for men under the age of 50 in the UK - and sport is seen as an effective way of communicating that help is available.

"My hope is that every sporting organisation in the country will look at what is being launched today and ask a question: 'What more can we do?' Because the opportunity before us is enormous," Prince William said.

The royal couple saw a bench at the ground dedicated in memory of Gary Speed, a footballer who once played at the club, and who died by suicide in 2011.

The bench, unveiled last year, is intended to give people a space to speak openly about their anxieties and the royal couple met Speed's mother there.

Shearer was a friend of Speed and is backing the call for people to try to support people they have worries about.

"You don't have to be an expert. You don't have to have all the answers," said Shearer. "But we can all be better at asking, listening, and helping someone find the right support."