The hackers claimed they have a staggering 756 gigabytes of information, including those pertaining to many A-list music and entertainment figures, such as Lady Gaga, Madonna, Nicki Minaj, Bruce Springsteen and Mariah Carey.

According to cybersecurity firm Emsisoft, the hackers said, in a post on a dark web forum, the information includes contracts, non-disclosure agreements, contact details and private correspondence.

Calling themselves REvil, the hackers have threatened to auction off the stolen documents belonging to the firm's top clients in another post on the dark web.

Laying out a schedule beginning on July 1, they said the first to be sold will be documents pertaining to Mariah Carey, Nicki Minaj, and Lebron James. All three start at US$600,000.

The second set of documents will be auctioned off two days later, on July 3, pertaining to music company Bad Boy Records, MTV, and Universal.

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Bidding for Bad Boy begins at US$750,000, while documents for MTV and Universal start at US$1 million.

An unspecified third set of documents will be auctioned off on July 5.

The note reads in part: "We have so many value files, and the lucky ones who buy these data will be satisfied for a very long time. Show business is not concerts and love of fans only - also it is big money and social manipulation, mud lurking behind the scenes and sexual scandals, drugs and treachery."

The note ends with a message to the law firm's founder, famed entertainment lawyer Allen Grubman: "Mr Grubman, you have a chance to stop that, and you know what to do."

This is presumably a reference to an earlier US$42 million ransom request.

Mr Grubman has maintained that he would not negotiate with the hackers. A statement by a spokesperson for the firm said: "The most recent post is yet another desperate nuisance tactic these criminals are using to try to squeeze out a profit from stolen data. Our clients and the entertainment industry as a whole have overwhelmingly applauded the firm's position that we will not give into extortion."