In a landmark case, Hogan Lovells has successfully argued that sensitive documents created by lawyers and other advisers during an internal investigation into mining company Eurasian Natural Resources Corporation (ENRC) are protected by litigation privilege.

The Court of Appeal decision will have significant consequences for disputes lawyers across the UK, and reassures both large and small businesses that they can obtain legal advice on sensitive matters without concerns over confidentiality.

However, the case also revives questions about what constitutes a client for the purposes of legal advice privilege.

Hogan Lovells commercial litigation partner Michael Roberts, who represented ENRC, said of the decision: "This historic ruling by the Court of Appeal is significant not just for ENRC but for any company faced with undertaking an internal investigation in response to a whistleblower or other allegation of wrongdoing.

"It is critical that companies are not penalised for acting responsibly, and are able to instruct lawyers to conduct investigations without fear that the authorities will later be able to demand all of the lawyers' work product. Following this ruling, it will remain for the company to decide whether, and to what extent, it is prepared to waive privilege."

Roberts was supported by Fountain Court Chambers' Bankim Thanki QC, Tamara Oppenheimer and Rebecca Loveridge.

The Court of Appeal judgment overturns a 2017 High Court decision that found in favour of the Serious Fraud Office (SFO), which first brought the claim in 2016. The SFO had argued that ENRC's claims to privilege over documents containing notes on interviews with current and former employees were unfounded.

High Court judge Ms Justice Andrews agreed with the SFO, ruling that the documents were not protected by privilege – a judgment that cast significant doubts over the robustness of litigation privilege.

Law Society president Christina Blacklaws warned of the dangers inherent in this: "If the High Court ruling had been upheld, any organisation facing a prosecution – not just multinationals, but charities, newspapers, small businesses or local authorities – could have to turn over private communications with their lawyers."

The controversy started in 2010, after allegations of corruption emerged at FTSE 100-listed ENRC that entailed both bribery and financial malpractice. The mining company sought counsel from US firm Dechert, which in 2011 took a lead role on the internal investigation that soon followed. Dechert's global white-collar co-head Neil Gerrard headed the investigation, having brought over the mandate following his move from DLA Piper that same year.

However, the relationship soon deteriorated and in March 2013, ENRC fired Dechert, precipitating an "acrimonious" split. The wrangle crescendoed last year when ENRC accused Dechert of leaking confidential information to The Sunday Times.

The Law Society saw fit to intervene in the case to address "matters of principle" raised by the original High Court ruling, and sought advice from Reed Smith white-collar crime partner Eoin O'Shea. Additional counsel was provided by Dinah Rose QC and David Pievsky of Blackstone Chambers.

O'Shea commented: "The law of privilege is a vital element of the rule of law and a precondition for proper access to justice. The result is welcome indeed. It amounts to a recalibration of the law of privilege, moving it in a more realistic and principled direction.

"Privilege now clearly applies in circumstances where there is a genuine concern about future prosecution and there is no longer any artificial distinction between civil and criminal proceedings."

Eversheds Sutherland represented the SFO with a team led by partner Peter Jones, who instructed Red Lion Chambers' Jonathan Fisher QC and Blackstone Chambers' Eesvan Krishnan and James Segan.

For Clifford Chance's (CC) London head of litigation and dispute resolution Helen Carty, the reversal by the Court of Appeal introduces "the certainty that was missing before the decision", for both advisers and corporates.

The Court of Appeal also accepted that the problematic definition of "client" in matters concerning privilege is in need of review – an issue that first came to light in 2004 in the Three Rivers case, which narrowed the scope of who qualifies as a client for privilege purposes.

Carty commented: "This would need to go to Supreme Court to litigate on the Three Rivers decision. But the Court of Appeal did go into some detail about [its] views on the issue, and said, if it had been open to [it] to depart from Three Rivers, [it] would have been in favour of doing so."

Julian Acratopulo, head of CC's international commercial litigation group and president of the London Solicitors Litigation Association, added that "lawyers and clients investigating the most serious criminal issues will sleep a little easier", but that they "may well still have nightmares about what constitutes a client for the purposes of legal advice privilege".

Ropes & Gray London international risk practice co-head Judith Seddon (pictured) described the decision as a "vital acknowledgment that legal professional privilege is not merely a rule of evidence but a substantive and fundamental right, and an important check on the power of the state".

"As a result of today's decision, individuals and companies, no matter how large or small, can once again be assured that when dealing with complex issues of alleged corporate wrongdoing, they are able to engage freely and frankly with their legal counsel and other advisers.

"Given that the inability to assert privilege in one jurisdiction can have – often severe – collateral ramifications in others, the significance of this decision will stretch well beyond the UK."