The king of pop: Jackson, pictured in 1999, had his song rights sold to Sony
In April the details of 77million gamers were stolen after Sony's Playstation Network was hacked. The breach cost Sony £106million and hugely damaged their reputation.
The attack on the Jackson files occurred shortly afterwards but has not been revealed until now.
The hack was discovered during routine monitoring of social networking sites, Jackson fan sites and hacking forums.
On the roster: Olly Murs also had work compromised
A source close to the company said: 'Everything Sony purchased from the Michael Jackson estate was compromised.
'It caused them to check their systems and they found the breach. There was a degree of sophistication.
'Sony identified the weakness and plugged the gap.'
The hack has compromised the work of other artists managed by the firm, including songs by Jimi Hendrix, Paul Simon, Olly Murs, the Foo Fighters and Avril Lavigne.
The source added that the second breach happening so soon after the first 'would have made investors and artists think, 'What other part of Sony isn't secure?''
Last night Sony admitted there had been a security breach and that the Michael Jackson material had been stolen but refused to say how much the hackers downloaded.
A source within the company said that although the Jackson estate had been told about the hack the company did not have to make the knowledge public as there was no customer data involved.
They added that computer experts had traced the hack to the UK by examining a 'fingerprint' allegedly left behind.
The Serious Organised Crime division took up the case and two men appeared in court last week charged with offences under the computer Misuse Act.
They denied all charges and were remanded on bail.
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