Behind the chirpy, sunshiny public image of Catherine, the Duchess of Cambridge, is a mother of two who has felt lonely at times.

This revelation came when the 35-year-old spoke to the founders of an app called Mush during a visit to a London high school with her husband, Prince William, and brother-in-law, Prince Harry, on Thursday.

"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," she confided to the women who had co-launched Mush to link up mothers.

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

Media reports said the mother of Prince George, three, and Princess Charlotte, almost two, had also praised Prince Harry the day before for admitting that he had spent 20 years bottling up his emotions after the death of his mother, Princess Diana, in a 1997 car crash.

"We all felt there was very much a campaign to get everyone talking and have these conversations (so) we've got to do the same... he's been brilliant," she noted, while hosting runners for the Virgin Money London Marathon at Kensington Palace.

Earlier this week, Prince William also got help from Lady Gaga to urge people to open up about mental health issues.

He put out a video on Tuesday in which he spoke with the American pop star, whose 2011 hit Born This Way celebrates that everyone can take pride in being free and a distinct individual.