Pop superstar Cher has defended her decision to lip-sync a performance on U.S. Tv after the awkward moment sparked an online backlash.

The singer appeared on The Ellen DeGeneres Show with her mother Georgia Holt to promote the 87 year old's new album, and they teamed up to perform a duet called I'm Just Your Yesterday which features on the record.

The pair held microphones as they shakily mimed along to a backing track, prompting a barrage of online criticism, but Cher insists they had to lip-sync the song because of her mother's recent battle with pneumonia.

In a series of posts on Twitter.com, she writes, "Yesterday my 87 yr old mom (sick for 2 months, & 13 lbs lighter) worked from 11 in the morning till 11 at night!... Tired, but B4 (before) I go... Let's put this lip sync thing 2 (to) bed, once & 4 (for) all. I wasn't sure mom would B (be) here 4 (for) Ellen, who gives a fk (f**k) if she sang live."