US singer Britney Spears has surprised fans and dropped a previously unreleased single called 'Mood Ring'.

The 38 year-old announced that the bonus track from her 2016 album Glory would be available to stream on all platforms.

The song was initially only a bonus track on the Japanese edition of her album. 

After not releasing music in four years, she wrote on Twitter: “Repurposed this since we didn’t use it: You folks wanted a new album cover ….. ta da there you go!!!! What was requested next is out now ….. I hope you turn #MoodRing up sooooooo loud.”

The news comes weeks after she released a new cover for the album when it shot to No 1 on iTunes.

Fans have gone wild: “I have been waiting for this since the album release. Thank you Queen Britney during these times we need new music from you. Love you," said one.

As another wrote: “Thanks for the gift! This song is a gem, we can’t wait for what’s next queen!”