The Lulzsec hacking group, which this week claimed to have obtained 4GB of emails taken from the Sun's servers, has decided not to publish them for fear of jeopardising ongoing legal actions in the UK and US.

However, an expert in email security procedures has warned the Guardian that the hack itself could undermine ongoing legal cases.

In a tweet sent from the @anonymousIRC account, which has 124,000 followers, a spokesman for the group said: "We think, actually we may not release emails from the Sun, simply because it may compromise the court case."

This may not be enough to prevent News Corporation, parent company of Sun publisher News International, challenging the admissibility of email evidence in future court cases, according to one information security professional.

"Post-Enron, new US laws were passed requiring all corporations to keep immutable email archives for legal compliance purposes. These are often provided by independent third parties," he said.

"Depending on whether Lulzsec got their material from this archive, rather than an old News International server, it's possible News Corporation will be able to argue their email archive can't be trusted in court as it's been compromised."

The claimed email cache is said to have been obtained after a hacking attack against News International on Tuesday night during which members of LulzSec apparently broke into computer systems there and redirected readers of the Sun's website to a faked page claiming News Corp chief executive Rupert Murdoch had been found dead.

Different members of the wider Anonymous hacking collective claimed the emails were taken from a seperate hack of News International's offsite backup centre in India. Establishing from exactly which server – if any – emails were obtained could prove crucial to keeping email admissable in a future court case.

Some accounts belonging to Anonymous also began tweeting email addresses and passwords for staff at News International, including what seemed to be an email account and password for Rebekah Brooks under her previous married name of Wade while at the Sun.

The password appeared to be valid based on the contents of the tweet, which included the encrypted form of the password.

News International reacted by closing down all external access to its webmail systems and forcing users to reset their passwords.

The company declined to comment at the time on whether the hackers might have had external access to email accounts, but the fact that it shut down the access suggests that it feared they might.

Equally, the hackers almost certainly would not have begun tweeting details of their find without having first exploited it.

Contacts within Anonymous have told Guardian journalists that News International's email systems were being probed last week and that downloads were being made then.

Lulzsec said they would now seek to release extracts of the emails they collected through media outlets.

"We're currently working with certain media outlets who have been granted exclusive access to some of the News of the World emails we have," said a tweet from the official @Lulzsec feed.

In a separate development, the legal firm that Rupert Murdoch accused of making a "major mistake" in its part in the internal investigation into phone hacking is to be called before a parliamentary select committee to defend itself against allegations that it helped "cover up" the scandal.

The culture, media, and sport committee, which grilled the Murdochs, is to write to Harbottle & Lewis, asking it to appear when hearings resume. Paul Farrelly, one of the MPs on the committee, said they "stand right up there with all the other people who have come to us and maintained there was only one rogue reporter."

He said a letter written by the law firm in 2007 to NI concerning a review of internal News of the World emails was now "clearly misleading".