The Royal Foundation focuses on issues such as helping young people, wildlife conservation, cyberbullying and supporting the military, most notably through the Invictus Games for wounded or disabled veterans which Harry founded.

Last year, the princes were widely praised for leading a campaign to tackle the stigma of mental health illness and speaking out about how they had personally suffered following the death of their mother Princess Diana in a Paris car crash in 1997.

"We're pretty tied up with planning a wedding at the moment but we're really looking forward to working as a pair and as a four going forward and hoping to make as much of a difference where we can," Harry said.

The four were pictured together leaving a service on Christmas Day on the queen's Sandringham estate but the event on Wednesday marked the first time the group - dubbed the "Fab Four" by media - had all attended an official event.

Asked if there were any disagreements with the family working closely together, William joked: "Oh, yes!"

Harry added: "They come so thick and fast. Working as family does have its challenges, of course it does. But we're stuck together for the rest of our lives."

Reuters