Robbie Williams has been revealed as a brand ambassador for WW, formerly known as Weight Watchers, alongside Oprah Winfrey and Kate Hudson.
The X Factor judge will use his personal weight journey to encourage others to be healthier in 2019.
He is one of several ambassadors in the Every Body Matters campaign.
Weight Watchers rebranded as WW, dropping the 'Weight' reference, in September and now aims to focus on wellness and health.
Williams will be sharing his "wellness journey" on social media, using the hashtag #MyWhy.
He said: "WW is the first programme that truly feels like it fits with my life.
"After my back injury last year, I definitely let my healthy habits slip and I felt as though I let myself and my family down.
"I can't keep trying the same things and hoping for a different outcome.
"I trust the history of behaviour science that WW has, and I'm proud to be a new ambassador. Hopefully, others will benefit from my experience."
The campaign is "based on the insight that everyone has their own reason to get healthy and what makes the most meaningful impact along the journey is everything you gain", WW said.
Williams has previously spoken about his battle with his weight and poked fun at himself on his 2013 album with a song called No One Likes A Fat Pop Star.
His wife, actor Ayda Field, has also spoken about the singer's battles with image, saying being branded "the fat dancer from Take That" when he was 15 stuck with him throughout his adult life.
Last year, Williams joked that he was putting on weight because of a lack of sex with strangers.
He added it had been replaced by "cake and carbohydrates".
And he had to battle a night-sleep disorder which saw him get up in his sleep and raid the fridge, claiming he wanted sugar not kale when that happened.
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According to WW research, 90% of adults across the world will set "improving physical, mental and emotional health" as a goal for 2019.
The WW campaign will launch on Boxing Day.