Britney Spears has surprised fans by announcing she's releasing unreleased 'Glory' track 'Mood Ring'.

The 'Oops... I Did It Again' hitmaker has decided to put the bonus track from the Japanese copy of her 2016 album on streaming services, four years after the LP's release, as a special treat for her fanbase..

Alongside a clip of the track, the pop superstar tweeted: ''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 !!!! (sic)''

On the steamy song, Britney sings: ''My love is a mood ring, Up and down emotions.

''All these mood swings, you know how to read the touch of my skin.

''Nothing on my body but this mood ring, you change me.''

Jon Asher, who co-wrote the song, recently revealed on a live-stream that it was intended to be for electronic duo The Chainsmokers featuring Britney.

However, the 'Paris' hitmakers decided it wasn't for them.

The repurposed song comes after Britney shared an unreleased version of the 'Glory' album cover.

She said: ''You asked for a new Glory cover and since it went to number one we had to make it happen!!!!

''Couldn't have done it without you all.''

Meanwhile, Britney recently marked the 20th anniversary of her iconic album, 'Oops... I Did It Again'.

Making a nod to the hit track 'Lucky', she wrote: ''All of my expectations were exceeded!!!! And it's all thanks to you folks.

''Thank you for sticking with me and growing with me. I am one Lucky girl.''

'Mood Ring' will be available to stream as of midnight.