The Duke and Duchess of Cambridge and the Duke and Duchess of Sussex have teamed up to launch a text messaging service for people experiencing a mental health crisis.
William, Kate, Meghan and Harry have backed the initiative, called Shout, with £3m from their Royal Foundation.
The free, anonymous service connects people experiencing a "tough moment" with trained volunteers.
Prince William appealed for more people to work as a volunteer for Shout.
The service hopes to have 4,000 volunteers by the end of the year.
Prince William said Shout provides "instant support," adding: "You can have a conversation anywhere and anytime - at school, at home, on the bus, anywhere."
During a 12-month pilot last year, 1,000 volunteers signed up to the initiative run by the charity Mental Health Innovations and 60,000 conversations took place.
Research by Shout suggests 85% of the texts received during the pilot were sent by people aged 25 or younger.
"That is 60,000 moments when people who were feeling scared, frightened and alone were able to use their phone to connect with someone who could support them," Prince William said at a launch event at Kensington Palace.
Shout aims to help people experiencing problems - from suicidal thoughts to bullying and relationship issues - move from "crisis to calm".
The Duchess of Cambridge said: "For the last few years, I've been focusing much of my work on the importance of prevention in the earliest years of life to help avoid problems in later life.
"But, sadly, for so many, they have already reached a crisis situation."
Kate added that Shout offered crucial support and "the opportunity to turn lives around".
Shout was researched and developed by the Royal Foundation, a charity the royal couples set up together.
The service operates 24 hours a day, seven days a week and is modelled on the US-based Crisis Text Line, which was launched in August 2013.
The volunteers are supported by clinically-trained supervisors. They need to be over 18, complete 25 hours of online training and commit to between two and four hours volunteering each week.
Lorraine Heggessey, the Royal Foundation's chief executive officer, said the "innovative solution" helps tackle "one of today's biggest challenges - the increasing number of people needing mental health support".
The Institute of Global Health Innovation (IGHI) at Imperial College London will work with the project to identify trends and develop insights into mental health to shape the provision of services.
The royals have already been campaigning around the issue and in 2016 launched their Heads Together campaign to end the stigma around mental health.
The charity running Shout also received a £1.5m grant from BBC Children in Need.