Sia has lavished praise on Britney Spears' singing as she works on her new album.

The pop princess has been holed away for months as she creates the follow-up to her 2011 record 'Femme Fatale' and Spears previously revealed that songwriter Sia is helping mould the album.

The Australian hitmaker, who has previously written tracks like Rihanna's 'Diamonds', tweeted today (August 20):

"@britneyspears your vocal on this tune is outstanding. WERK!"

Spears meanwhile has launched an online countdown clock ahead of an apparent big announcement on September 17.

It has been suggested that the 'Toxic' star will then unveil her long-awaited comeback single.