Rebekah Brooks is facing a legal battle over new allegations that phone hacking was “endemic” when she was editor of The Sun, a court has heard.

Lawyers for News Group Newspapers, a division of Rupert Murdoch’s UK print business, told a High Court hearing that a “new flank” of hacking claims had been opened against Rupert Murdoch’s daily tabloid.

Ms Brooks, the chief executive of News UK, who resigned from the same post shortly after the closure of the News of the World in 2011, was found not guilty of involvement in phone hacking after a lengthy criminal trial, which ended in 2014. She was reappointed to her old job last year.

The potential inclusion of The Sun in the third tranche of phone-hacking civil actions – which had previously all related to its sister paper the NOTW – could leave the company facing substantial extra compensation payouts.

News UK, the rebranded name of News International, is reported to have already spent more than £300m in the fallout from phone hacking at the NOTW. The total cost of defending the Murdoch empire is thought to be close to $1bn (£690m). NGN has always said that there was no hacking activity at The Sun.

Hacking trial: The verdicts in full Show all 7 1 /7 Hacking trial: The verdicts in full Hacking trial: The verdicts in full Rebekah Brooks The former News of the World editor and News International chief executive has been cleared of conspiracy to hack phones; misconduct in public office for allegedly signing off payments to a Sun journalist's 'number one military contact' between 2004 and 2012; conspiracy to pervert the course of justice after seven boxes were allegedly removed from the NI archive just days before 2011 arrests Hacking trial: The verdicts in full Andy Coulson Former News of the World editor and Downing Street spin doctor guilty of conspiracy to hack phones from 2000 to 2006. The jury failed to reach a majority verdict on charges of conspiracy to commit misconduct in a public office by allegedly paying police officers for two royal directories. He could face a retrial. Hacking trial: The verdicts in full Stuart Kuttner Retired managing editor cleared of involvement in phone-hacking conspiracy spanning six years Reuters Hacking trial: The verdicts in full Cheryl Carter Brooks' former personal assistant, cleared of conspiracy to pervert the course of justice by removing seven boxes from the News International company archive just days before she was arrested in 2011 Hacking trial: The verdicts in full Charlie Brooks Racehorse trainer and Rebekah Brooks' husband, cleared of perverting the course of justice around the time of police searches in July 2011 Hacking trial: The verdicts in full Mark Hanna Former News International director of security, cleared of perverting the course of justice Hacking trial: The verdicts in full Clive Goodman The former News of the World royal editor, could face a retrial after the jury failed to reach a verdict on charges of committing misconduct in public office for allegedly paying police officers for two royal directories

A civil trial over the latest claims against the NOTW had been scheduled for April, with a selection of test cases from a possible 16 claims pencilled in. The court heard that a further 25 were also pending.

If five of the claimants are successful and they are allowed to amend their claims to include phone hacking at to include phone hacking at The Sun, lawyers representing victims say a potential 60 cases could follow. News UK says such claims are unsubstantiated and will be vigorously challenged in court.

Claimants alleging that hacking took place at The Sun under Ms Brooks’ watch are relying on witness statements from former NGN journalists and the private investigator regularly commissioned to hack by the NOTW. In documents lodged with the court, several witnesses maintain they were “intimately involved or engaged in these unlawful activities on behalf of NGN, both at the News of the World and The Sun”.

News Group Newspapers has always said that there was no hacking activity at The Sun (Getty) (Getty Images)

Greg Miskiw, the former assistant editor of the NOTW while Ms Brooks was at the helm until 2002, confirms in his statement that “phone hacking and blagging were used and that this was known about by Ms Brooks”.

Paul McMullan, who worked at both The Sun and the NOTW, as a features writer and features editor appointed by Ms Brooks up until 2001, alleges the widespread use of voicemail interceptions at both papers.

Stuart Hoare, the brother of the late Sean Hoare, who worked on both NGN tabloids, said his brother “confessed before he died to hacking phones with Dominic Mohan”, then showbiz editor of The Sun, who went on to edit the paper. He claims the practice was “endemic” within the two newspapers.

Glenn Mulcaire, the private investigator jailed for phone hacking, said that he was also hired to provide unlawfully obtained information for The Sun or for articles that appeared in the daily paper.

Lawyers for the new claimants maintain that phone hacking was used by The Sun in published stories and that they have a “strong case based on the evidence of Ms Brooks’ knowledge and approval of such activities”.

Video: Brooks speaks after being cleared of phone hacking

Among the new claimants is Simon Clegg, a former chief executive of the British Olympic Association. He was a key individual in persuading the UK government to bid for the 2012 Games.

Mr Clegg was notified by the Metropolitan Police in 2014 that he was a victim of phone hacking. In his statement, lodged with the court, he claims to have identified 11 articles that were “examples of voicemail interception” used for stories published in the NOTW and The Sun.

Although NGN has been aware of Mr Clegg’s case since last year, he said the company had offered no explanation about how they obtained information for the articles. One of the cited articles from 2002 was headlined “Olympic bid hits a hurdle” and offered information from a “source” which Mr Clegg said could only have been his voicemail.

Video: Rebekah Brooks back to work at News UK

EastEnders actors Christopher Parker and Brooke Kinsella and Coronation Street actors Kym Marsh, Samia Ghadie and Alan Halsall also feature in the list.

Others are designer Pearl Lowe and her musician husband Danny Goffey and actor and comedian Les Dennis.

The judge, Mr Justice Mann, granted an adjournment to NGN on hearing the full details of The Sun cases.

A spokesperson for News UK said: “Following many years of investigation, there were no charges against The Sun or its employees for voicemail interception.

“Today, certain claimants seeking financial settlements arising from activities at the News of the World have made unsubstantiated claims against The Sun. If the court permits such claims to proceed, The Sun will defend them vigorously.”