The pop superstar hit headlines across the globe over the summer (15) when she wrote an open letter to the firm accusing bosses of short-changing artists by refusing to pay royalties during a three-month free trial period for subscribers.

Apple chiefs later reversed their decision in a landmark victory for Taylor, but the singer has now revealed she feared the stunt could backfire on her.

Taylor made the admission during an interview with DJ Zane Lowe on Apple Music's Beats 1 Radio to promote a new concert film she is releasing via Apple Music this month (Dec15).

"I was struck with this overwhelming sense of fear," she tells the host. "Like, are they going to turn my phone off? Are they going to turn the video camera on? Are they watching me right now? Am I going to wake up tomorrow and all of my music will be off of iTunes? Like, absolute terror hit."

Taylor's new film is directed by Jonas Akerlund and features footage from a show on her 1989 World Tour concert in New Zealand which was filmed last month (Nov15).

The footage will be available on Apple's streaming service from 20 December (15) and features backstage clips, tour rehearsals, and appearances by special guests who have joined the pop star on stage during the trek, including Mick Jagger, Justin Timberlake and Selena Gomez.

"Getting ready for this I wondered about a lot of things," Taylor declares in the trailer for the film. "I wondered what it would be like to perform for 70,000 people. I wondered what would happen if I invited the most amazing artists in the world to come out with me and perform on my stage - would they do it?"

The news was announced as Taylor celebrated her 26th birthday on Sunday (13Dec15), just a day after she wrapped her seven-month 1989 tour in Melbourne, Australia on Saturday (12Dec15).