The Duke of Cambridge has spoken of his shock at being called out to his first suicide as an air ambulance pilot in a joint interview with his brother, Prince Harry, on tackling masculinity and mental health issues.

Prince William said the “tipping point” for him in understanding how men struggle to deal with mental health problems came with his work with the East Anglian air ambulance.

“My first callout was to a male suicide and I was told there were five suicides or attempted suicides every day in East Anglia alone. When I looked into it I was shocked by how bad this situation is – suicide is the biggest killer of men under 45 in the UK – which is absolutely appalling.”

He praised grime star Stormzy, sportsmen Rio Ferdinand and Freddie Flintoff, and rapper Professor Green for publicly talking about pressures on their mental health.

The interview with the princes is part of a campaign to encourage better communication about mental health issues. It follows Harry’s revelations that he sought counselling after coming close to a breakdown over the death of his mother and had two years of “total chaos” that led him to process his grief.

The interview, with CALMzine, published by the Campaign Against Living Miserably, is for a special edition before the London Marathon, during which the princes and the Duchess of Cambridge will cheer on runners taking part for their mental health campaign, Heads Together, which is the charity of the year for the marathon.

Talking about how men often feel it was a sign of weakness to discuss their mental health, William said: “There may be a time and a place for the stiff upper lip, but not at the expense of your health. The recent interview by Stormzy about his depression was incredibly powerful and will help young men feel that it’s a sign of strength to talk about and look after your mind as well as your body.

He said he hoped subsequent generations would find it more normal to talk about their emotions. “Catherine and I are clear that we want both George and Charlotte to grow up feeling able to talk about their emotions and feelings.”

Harry, who served two tours of Afghanistan with the army and who actively campaigned to raise awareness of mental health issues within the military, believed progress was being made.

“The military is a complex picture as on one hand there is an incredible sense of brotherhood and belonging between you and your mates,” he told the magazine. “You’ll do anything for each other – scrub each other’s boots, drag each other through the mud – anything.

“Yet, on the other hand, this support for each other hasn’t, up to now, included looking after how your buddy is feeling and thinking about things. When you’re serving you look after your physical health, your training, your equipment, but not your head.

“There’s definitely been a misplaced sense of pride has got in the way of the people in the military community talking about their mental health and getting help. It’s changing now and I’m proud that this is part of the Heads Together campaign.

“Hopefully, if men see soldiers talking about mental health, it will give them the confidence to do the same.”

In an interview with the Daily Telegraph published on Monday, the prince reflected on the impact of his mother’s death and the time it took him to process it. “[I thought] it’s only going to make you sad, it’s not going to bring her back,” he said. “So from an emotional side, I was like ‘right, don’t ever let your emotions be part of anything’. So I was a typical 20-, 25-, 28-year-old running around going ‘life is great’, or ‘life is fine’ and that was exactly it.

“And then [I] started to have a few conversations and actually all of a sudden, all of this grief that I have never processed started to come to the forefront and I was like, there is actually a lot of stuff here that I need to deal with.”

On 19 April, four days before the marathon, the duchess will host runners from Team Heads Together at Kensington Palace to talk about their reasons for running and wish them luck.

William will attend the screening of the BBC’s Mind Over Marathon documentary at the BBC Radio Theatre on 18 April, which follows 10 runners affected by mental health issues. The royal trio will open the Global Academy in support of Heads Together on 20 April.

• In the UK, the Samaritans can be contacted on 116 123. In the US, the National Suicide Prevention Hotline is 1-800-273-8255. In Australia, the crisis support service Lifeline is on 13 11 14.