Justin Bieber has accused Taylor Swift of "crossing a line" after she labelled Scooter Braun a "bully" on social media, amid the music mogul acquiring her entire back catalogue.

Braun has agreed to purchase Scott Borchetta's record company, including its Big Machine Music publishing operation, through his Ithaca Holdings firm for more than $300 million (£236.2 million) - meaning he will own Swift's masters for each project released on her former label, from her 2006 self-titled debut up to 2017's Reputation.

Swift hit back in a lengthy post on her Tumblr page on Sunday, admitting she feels "betrayed" and "grossed out" by the sale, which she claimed she wasn't made aware of until the news was made public in Braun's statement.

She also alleged Braun "got his two clients together to bully" her about the headline-grabbing incidents, posting a screenshot of a Facetime call Bieber once had with his manager and Kanye West, which he had posted online at the time alongside the message, "Taylor swift what up."

Shortly after Swift's Tumblr post, Bieber responded in a message of his own - apologising for his part in the bullying but blasting the Shake It Off star for taking aim so publicly at Braun.

"First of all i would like to apologise for posting that hurtful Instagram post, at the time i thought it was funny but looking back it was distasteful and insensitive," the Sorry singer wrote, insisting that Braun "didnt have anything to do with it (the post) and it wasnt even a part of the conversation in all actuality he was the person who told me not to joke like that (sic)."

"As the years have passed we haven't crossed paths and gotten to communicate our differences, hurts or frustrations," Bieber continued. "So for you to take it to social media and get people to hate on scooter isn't fair. What were you trying to accomplish by posting that blog? seems to me like it was to get sympathy u also knew that in posting that your fans would go and bully scooter."

Borchetta also called Swift out for some alleged inaccuracies in a statement on the Big Machine Music label website, including where the singer had claimed Borchetta had proposed during contract renegotiations that she "'earn' one album (master) back at a time, one for every new one I turned in".

"I walked away because I knew once I signed that contract, Scott Borchetta would sell the label, thereby selling me and my future," Swift added.

But Borchetta took issue with that claim, and wrote in his response: "100 per cent of all Taylor Swift assets were to be transferred to her immediately upon signing the new agreement. We were working together on a new type of deal for our new streaming world that was not necessarily tied to 'albums' but more of a length of time... Taylor had every chance in the world to own not just her master recordings, but every video, photograph, everything associated to her career. She chose to leave."

Swift left Big Machine Music to sign a new deal with Universal Music Group last November.

Since the 29-year-old published her Tumblr post, she has received messages of support from stars including Halsey, Cara Delevingne and Todrick Hall, while Demi Lovato and Braun's wife Yael Cohen are among those who have spoken out in support of the mogul.