Lady GaGa is blocking Cher from releasing her duet with her.

Gaga donated a track she wrote, 'The Greatest Thing', to the 67-year-old diva, which they then recorded together, but the results have never been aired.

When asked by a fan on twitter why the song has not been released the song, Cher wrote: ''It's done & she [Gaga] doesn't like it, or want it to come out. She's an artist, it's up to her. I'm disappointed 2 (sic).''

The producer behind the track - regular Gaga collaborator RedOne - previously said the song was something he and Gaga had worked on together, but remained unfinished until he played it to Cher.

He said: ''Gaga and I recorded the song a few years ago, and we were always in the studio enjoying that song. It's weird we never did anything with it.

''So I remember my management and Cher's management were trying to get us together, and I thought, 'Oh my God, I'd love to meet Cher, just to meet her!' And so I met her, I played her some songs, and I played her 'The Greatest Thing,' and she loved it. She said, 'This is my song.' ''

Originally Gaga was happy for Cher to record the track and to appear on the remix, but with RedOne's encouragement it became a duet.

He added: ''I cut the vocals with Cher first, and I was in Madrid and got a text from Gaga saying, 'I'm thinking about jumping on the song in the remix.' ''

''I was like, 'No, you should jump on the original! Make it a duet.' She said, 'Oh yes, OK, absolutely,' and then I spoke to Cher, and it became a party.''

Cher is set to release her 26th studio album, 'Closer To The Truth' - her first since 2001 - in September.