Declan Donnelly has thanked fans for their support for TV partner Ant McPartlin amid reports he has checked into rehab to fight prescription drug and alcohol problems.
McPartlin says he has "let people down" and wanted to speak out about his issues so he could help others.
The 41-year-old star, one half of presenting duo Ant and Dec, told The Sun on Sunday: "I feel like I have let a lot of people down and for that I am truly sorry.
"I want to thank my wife, family and closest friends for helping me through this really difficult time.
"I've spoken out because I think it's important that people ask for help if they're going through a rough time and get the proper treatment to help their recovery."
McPartlin is believed to have checked into a rehabilitation facility, where he will remain for up to two months.
The paper reported that the star's problems were triggered by failed knee surgery two years ago.
McPartlin has an ongoing knee problem and is said to have started taking prescription drugs to ease the pain.
A source said that alcohol had become a concern among his friends.
Meanwhile, Donnelly wrote on Ant and Dec's official Twitter account: "Thank you for all the kind messages and well wishes for the big fella.
"Your support is, as ever, much appreciated. DD xx."
McPartlin and Donnelly present programmes such as I'm A Celebrity ... Get Me Out Of Here! and Britain's Got Talent.
Ant and Dec started out starring in children's drama series Byker Grove in the 1990s and went on to pursue a music career under the names of their characters PJ & Duncan, releasing songs such as Let's Get Ready To Rhumble and Shout.
After presenting a range of programmes they began hosting Saturday morning ITV children's programmes SMTV Live and CD:UK, where they remained for three years from 1998 until 2001.
Fronting Pop Idol and Friends Like These catapulted them to prime-time fame in the early noughties, and they are now best-known for presenting I'm A Celebrity, Britain's Got Talent and their own entertainment show, Ant and Dec's Saturday Night Takeaway.
The duo have won the TV presenter award at the National Television Awards for 16 consecutive years.
Last year the two Geordies were awarded OBEs in the Queen's Birthday Honours.
McPartlin is married to his long-time partner Lisa Armstrong.