Liam Payne said he has "come a long way" since his One Direction debut as he released his first solo single following the group's temporary split.
The 23-year-old references the boy band in a verse of Strip That Down which was co-written by Ed Sheeran and features Quavo from hip-hop group Migos.
Payne sings: "You know I used to be in 1D, now I'm out free."
Explaining the single, which features a hypnotic bass synth, he said: "I've been working hard in the studio for over a year and I'm really excited to share this new music with my amazing fans.
"I've come a long way since our One Direction debut and I hope everyone likes my new sound as much as I do."
Payne, who recently became a father as girlfriend Cheryl gave birth to baby boy, added: "2017 has been a big year for me already and I'm looking forward to what the rest of it will bring."
Fans on social media were impressed by his debut solo effort but some were left "disappointed" by the One Direction lyric, as it hints the group's temporary split may be permanent.
Sonia posted on Twitter: "'I used to be in 1D now I'm free' @LiamPayne sit down and explain. You're in 1D. Don't play games."
Another fan wrote: "Strip That Down is amazing but what does mean 'You know, I used to be in 1D. Now I'm out, free?'"
Kaitlyn Rachelle wrote: "This lyric 'I used to be in 1D, now I'm out, free' in @LiamPayne new song #StripThatDown really broke my heart #disappointed."
Kaylee Clifford posted: "@LiamPayne is this you telling us you guys aren't getting back?"
Melissa B wrote: "What does it mean? You weren't happy in 1D? @LiamPayne we are your fans and we want to know."
One Direction took a hiatus in March last year which they promised fans would be temporary. Payne is the final member of the group to unveil his solo work since they took a break.