The film, directed by and starring Bradley Cooper, sees Gaga in the role of Ally, an aspiring singer helped into the industry by an older musician.

It's a remake of the 1976 Barbra Streisand and Kris Kristofferson film - itself a remake of two earlier versions, from 1954 and 1937.

Gaga said being in the film was a "dream come true and an honour".

The trailer, released on Wednesday, sees the Poker Face singer, known for her outrageous outfits, with a stripped-back look a world away from her stage persona.

It starts with Cooper, who plays singer Jackson Maine, performing in front of a huge audience - in scenes actually filmed on the Pyramid Stage at last year's Glastonbury Festival, where Cooper was on stage to introduce Kristofferson.

The trailer sees a new look for Lady Gaga

The Glastonbury crowd makes a brief appearance

Gaga praised Cooper's singing abilities in her tweet.

Meanwhile, the screenwriter and director Joss Whedon was one of many people to be impressed by Gaga's acting prowess.

Katy Perry said she was "screaming" at the trailer, while Lea Michele said she couldn't wait to see it.

Some are already suggesting that either Gaga - real name Stefani Germanotta - or Cooper, making his debut as a director, could be in line for an Oscar next year.

Original songs have been written for the film

Gaga has composed new songs for the film, which sees the couple fall in love before experiencing struggles as Ally's fame rises, while Jackson's falls. The original songs have been written alongside Cooper and other artists including Mark Ronson.

Judy Garland appeared with James Mason in the 1954 version of A Star is Born, while the 1937 film starred Janet Gaynor and Fredric March. The first two films were set in Hollywood, before the action moved to the music industry for the Streisand film.

A Star is Born is released by Warner Brothers in October.