TalkSafe will offer counselling for Londoners aged 10 to 18 facing issues like family breakdown, bullying and sexual relationship issues.
THT regional delivery manager for London Adam Wilkinson said: "TalkSafe aims to be a one-stop shop for young people where they can get accurate information on a range of topics."
Young Londoners will be able to access the service via the internet, their mobile phones or face-to-face with a trained mentor.
"The internet is one of the main places where teens get information so it's really important that there's a dedicated site for them that's accurate, reliable and safe."
The Big Lottery Fund awarded THT with more than £400,000 to launch TalkSafe, which hopes to link up with schools across London to raise awareness about the service.
MP Ann Keen and mayoral advisor on health and youth opportunities Pam Chesters will be attending the official launch on 6 April. Young people, teachers and youth organisations are also being invited to attend.
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Terrence Higgins Trust launches peer mentoring in London
A peer mentoring service is being launched for young people in the capital by sexual health charity Terrence Higgins Trust (THT).