Gwyneth Paltrow sings onstage at Holly Williams gig

Hollywood actress Gwyneth Paltrow stunned a theatre audience by joining folk star Holly Williams onstage to sing backing vocals during a show.

The Iron Man actress, who showed off her singing skills by adding harmonies to Williams' vocals on her 2013 album The Highway, helped her perform the tracks live at the Roxy Theatre in Los Angeles on Monday (28Apr14) night.

She remained onstage throughout the concert and was seen swaying along with the music.

Williams, the granddaughter of country legend Hank Williams, was playing a support slot for country rocker Jason Isbell.