Joker: Folie a Deux has made its debut in UK cinemas, but it's not all good news for the film as it has now marked an unfortunate milestone.

The sequel sees Joaquin Phoenix reprise his role from the 2019 original hit as Arthur Fleck, who is currently held in Arkham awaiting trial for his crimes committed in the first film.

During his time there, he encounters Lady Gaga's Harleen (or Lee) AKA Harley Quinn, who helps him find love and transforms the film into a full-blown musical.

The star-studded cast also includes Brendan Gleeson, Catherine Keener, Steve Coogan, Harry Lawtey and Bill Smitrovich. The film is directed by Todd Phillips, who was at the helm of the first film as well as The Hangover franchise and Starsky and Hutch.

However, upon its release, Joker 2 has seen its Rotten Tomatoes score take a significant tumble, reports the Mirror.

With a wave of new reviews being published online, the average rating for the movie starring Phoenix and Lady Gaga has plummeted to 39% from 159 critic reviews.

The newly revamped audience score on the website, now dubbed the Popcornmeter, doesn't paint a pretty picture either, standing at 37% from more than 500 verified ratings.

But what are critics saying about this audacious take on the comic book movie?

IGN, one of the websites reviewing the film, pointed out that one of the issues with the new film is its focus on what came before.

They shared: "The worst thing about Joker: Folie a Deux is its unfulfilled potential. It begins with the promise of a novel approach to the Joker and Harley Quinn, placing them in a world where the opposite of cruelty is musical romance.

"Unfortunately, the DC sequel gets bogged down by a lengthy courtroom saga, which not only keeps the dazzling Lady Gaga away from the spotlight, but centres the movie entirely around its own predecessor, without doing or saying anything new."

Total Film expressed disappointment as well: "Part courtroom movie, part behind-bars romance, Folie a Deux is an unconventional musical sequel that fails to hit the high notes."

Empire magazine offered a tepid warning to prospective viewers: "As sweet and beguiling a musical romance as it's possible to have between two murderous psychopaths. Its kooky approach won't suit all stripes of comic-book fan, but it finds a strange, tragic hopefulness all of its own."

However, not all reviews are negative; NME provided a more optimistic outlook, foreseeing potential for the series' future: "Phillips and Silver have delivered the last thing anyone expected: a socially responsible Joker movie that finds an intriguing way to explore the consequences (both on and offscreen) of the first film. Joker fans shouldn't cry too hard though Warner Bros. have cleverly found a way to leave the door open a little for the franchise to continue, should the need arise."

The Independent lauded Joaquin Phoenix's portrayal of the character in 'Joker', remarking: "The genius of it is that we can't help but care for Arthur despite his neediness and derangement. Even during the film's most apocalyptic and violent moments, we're always aware that, underneath Joker's gaudy war paint, lurks little, feeble Arthur. Against the odds, this ingenious and deeply unsettling film even turns into a bit of a weepie by the final reel."

Joker: Folie a Deux is in UK cinemas now.