Catherine joins William at suicide prevention campaign

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.

It's being supported by sport stars including Harry Kane, Dame Kelly Holmes and 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.

The project 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.

This event will be Prince William's biggest public appearance in the UK since the return of the Duke and Duchess of Sussex from the US.

Earlier this week, William attended the funeral of Norway's King Harald in Oslo and accompanied his son Prince George to his first day as a boarder at Eton College.

The level of suicide is disproportionately higher among men than women - it's 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 will say.

The royal couple will see 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 as part of efforts to tackle mental health challenges and loneliness.

Alan Shearer, former England football star and TV pundit, was a friend of Gary 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."

A film has been made, featuring England football captain Harry Kane and boxer Carl Frampton, talking about the importance of stepping in and offering help if people see someone who seems to be facing mental health problems.

Prince William helps to narrate the video, which wants to tackle the stigma around mental health and suicide, with 7,000 lives being lost to suicide each year in the UK.

The initiative has a practical toolkit, on a free website, for people who are worried or embarrassed about what to say to someone if they have concerns about their well-being.

It offers a sports-themed way of approaching the topic, called GOALS, representing G: get ready; O: open the conversation; A: ask the question; L: listen and respond and S: support and signpost.