WATCH: The Duke and Duchess of Cambridge and Prince Harry promote their Heads Together mental health campaign

The video was released today as the duchess followed in the footsteps of Prince Harry and made a heartfelt confession that motherhood has been "lonely" at times.

Kate, the mother of Prince George and Princess Charlotte, opened up during a visit to a west London high school, where she joined a discussion on the issue of mental health.

The group also included the Duke of Cambridge, Harry, students and Jamie Theakston and Emma Bunton, who had presented their Heart radio breakfast show from the Global Academy in Hayes.

Kate also suggested to the group that starting conversations about mental health with children should happen when they are younger.

She said parents should be helped "to start these conversations from a much earlier age but through play, even if their language hasn't yet developed".

After two mothers, supporters of the royal trio's Heads Together mental health campaign, discussed how they became friends after bonding through mutual loneliness, Kate replied: "Yes, it is lonely at times and you do feel quite isolated but actually so many other mothers are going through exactly what you are going through.

"It is being brave enough, like you obviously were, to reach out to those around you."

Kate was chatting to Katie Massie-Taylor and Sarah Hesz, who have launched an app to help mothers of young children make friends with others in a similar situation.

Commenting on the Duchess's admission, Ms Hesz said: "It's not surprising because we do know all mums feel that, and it doesn't matter where you live. It is something you go through when you become a mum and it's a challenge for everyone, whether you're royalty or a normal mum."

William, Kate and Harry have spent the past few days championing their Heads Together campaign, which aims to encourage people to talk about their mental health problems or be a sympathetic ear for someone in need.

On Wednesday, Harry spoke of his decision to talk out about the issues he faced after his mother's death, saying it was ''only right'' that he aired his experiences as he wanted to encourage others to ''smash that stigma'' around talking about mental health.

He was praised by the Duchess, who described him as ''brilliant'' for revealing in an interview with the Daily Telegraph that he spent nearly 20 years ''not thinking'' about the death of Diana, Princess of Wales, and eventually got help after two years of ''total chaos''.

The Global Academy, founded and operated by media and entertainment group Global, aims to prepare students for careers in the broadcast and digital media industries and welcomed its first pupils in September.

It was built on the site of the former home of British music company EMI, which is being transformed into a complex of shops, residential blocks and offices.

During his tour of the school, William listened to a short clip of a famous recording made by his great-grandfather, George VI - his message to the Empire on September 3 1939 after Prime Minister Neville Chamberlain had declared Britain was at war with Germany.

It was played to him by students who had made a documentary about vinyl recordings made by members of the Royal Family and held in the EMI archives.

Actor Colin Firth featured in the audio programme, reciting part of the same speech in his role as George VI in the film The King's Speech.

The Duke said: "It's fascinating, it really is. You can imagine Colin Firth sitting there trying to work out how on earth he's going to replicate it all."

Meanwhile, in a neighbouring classroom, Kate, wearing a red Armani suit, chatted to students learning about mental well-being, while Harry joined a class about the science of sound.

Parent company Global runs a number of radio stations, and breakfast presenters from its stations Heart, LBC and Capital recorded their morning shows from the school.

The royal trio watched as Heart's breakfast show was presented by Theakston and former Spice Girl Bunton.

William, Kate and Harry chatted to students helping the show's production team at a desk filled with an array of buttons, switches and lights.

The presenters were behind glass in a studio and William joked: "Funny thing to be locked in a box talking to a big red microphone", while Theakston quipped: "I'm not sure the royal party has been this close to a Spice Girl.''

After attending a school assembly, where they watched a student-produced film about depression and heard a moving poem on the same subject, William unveiled a microphone, created for George VI and featured in The King's Speech, to officially launch the academy.