Many viewers expressed disbelief it was really her. Photo / Supplied

Former No Doubt singer Gwen Stefani has caused a big stir on social media after she appeared on a US chat show this week.

The Hollaback Girl singer, who turns 53 next month, sat down for an interview on Late Night with Seth Meyers to discuss her current role as a coach on The Voice – but viewers instead wanted to talk about her "unrecognisable" appearance.

The always-glam star looked stunning, with long blonde hair and a bold fishnets-and-cut-outs ensemble - but her appearance had even some longtime fans of the singer asking if it was really her.

Clips from the interview posted to the show's YouTube account were flooded with hundreds of comments expressing surprise at Stefani's new look.

"She's literally unrecognisable," read one of the top-rated comments.

"She looks completely different now," said another.

"I'm old enough to remember her with pink hair and braces. Can't believe how different she looks now," said another, referring to one of her No Doubt looks of the late 90s.

"I'm a lifelong fan of Gwen's and I didn't recognise her. I'm so confused," wrote another.

"Would never have known this was Gwen Stefani without the title!" said another.

Stefani's eternally ageless looks have been the subject of much conjecture over the years, but the star has never spoken publicly about having any cosmetic procedures. In a 2016 Carpool Karaoke appearance, host James Corden asked her about how exactly she seemed to be ageing in reverse – and she attributed it to her relationship with now-husband Blake Shelton.

"I kind of started falling in love. Then I wrote a whole record about that. That's what the 'face' lift is, I think!" she said.

It's not the first time the chameleonic Stefani's made headlines for a near-unrecognisable new look. Back in 2016, she shocked fans by appearing at the Billboard Awards with vastly different make-up - enough of a major departure for her that her longtime make-up artist later had to publicly defend the new look.

And last year, the star caused a stir with a wild new hairdo as she fronted up to an episode of The Voice: her usual blonde hair, plus black bangs and a black ponytail.

Stefani evolved into a style icon after her early days as the frontwoman of 90s ska-pop band No Doubt, going solo in 2004 and eventually launching her own fashion and make-up lines.

But fans eager to hear more music from the star, who hasn't released an album in five years, will be disappointed by her candid confession in a recent interview.

Revealing she wrote "30+ songs" for a new album during Covid quarantine, Stefani confessed she didn't feel passionate about any of the material, and instead decided to focus her energies on her marriage to Shetlon.

"I kind of put it on the back-burner, but I also feel like sometimes, I don't really have much to say or I don't know if I do – and so, I'm just going to wait and pray about it," she told Forbes.