Justin Bieber may have cancelled the final leg of his Purpose World Tour, but he is showing no signs of slowing down. The pop superstar is set to appear in David Hasselhoff's new movie Killing Hasselhoff.

The Hoff admitted he wasn't "supposed to mention anything" before dropping the bombshell during an interview with Good Morning Britain on Wednesday (26 July).

The 65-year-old former Baywatch star told presenters Piers Morgan and Susanna Reid that Bieber makes a cameo in the final scene of the indie comedy, which also stars Ken Jeong, Jim Jeffries and Dan Bakkedahl.

Explaining how the cameo came about he said: "He's a friend of mine, you see. I did him a favour by doing a video that was never released and so he did me a favour by riding in the Knight Rider car at the end of the movie with me – it was quite a surprise."

Killing Hasselhoff tells the story of a man who finds himself involved in a high-stakes celebrity death pool and plots to murder David Hasselhoff after resorting to great lengths to try to pay off his debts. Bieber, whose film credits include 2012's Men In Black 3 and Zoolander 2, will feature as the voice of talking car KITT in the film.

As well as starring in it, Hasselhoff also produced the movie. "It's a comedy and I'm really proud of it. It took about two-and-a-half years to finally get released and now it's coming out on digital on September 18," he said.

Meanwhile, Bieber has broken his silence on his decision to cancel the remaining 14 dates of his Purpose world tour, telling TMZ that he was looking forward to some much-needed relaxation after performing a gruelling 150 dates in two years.

"I've been on tour for two years," Bieber he said in a video published by the site before apologising to fans he had disappointed. "I love you guys, I think you guys are awesome. I'm sorry for anyone who feels, like, disappointed or betrayed, it's not in my heart or anything and have a blessed day."

Watch the Killing Hasselhoff trailer here: