JUSTIN Bieber has apologised to fans after pulling out of a charity festival due to "unforeseen circumstances".

The pop star promised to make it up to his UK fans in a post on his Twitter site following the announcement by Fusion Festival, held at Birmingham's Cofton Park.

Bieber was due to headline the first day of the festival on Friday, August 28 and disappointed fans have been told they can have a refund.

In a note posted with the heading "UK beliebers I'm going to make this up to you", he said:

"To all my UK beliebers...Due to an unforeseen issue I will be unable to perform at the Fusion Festival on August 28th.

I support the Prince's Trust wholeheartedly and I am deeply disappointed I won't be able to perform.

"I look forward to the opportunity to give my UK beliebers a great show in the near future, and hope to honor my commitment to Trust at that time."

More than 50,000 people are expected at the park to see acts including McBusted, Ella Henderson, The Vamps and Rixton.

The festival said: "It is with great regret that, due to unforeseen circumstances, Justin Bieber has withdrawn from his performance at Fusion Festival on Friday 28th August 2015.

"We are now working hard to secure another exciting act for the festival and will hopefully have some news shortly.

"We will also announce details of how you can apply for a refund should you require one.

"Please stay tuned to our social media channels for further announcements soon."