Clifford Chance (CC) has been criticised by the Court of Appeal in its judgment in the long-running Excalibur case, a ruling that increases the exposure of litigation funders in cases that do not succeed.

The case involved a group of litigation funders that financed a $1.6bn case brought by Excalibur Ventures, an aspiring oil exploration firm, against two US oil companies.

Excalibur, which claimed that the defendants cheated it out of an exploration deal in Iraq, was represented by CC arbitration partner Alex Panayides, who was working on a partial contingency fee.

The Court of Appeal's ruling upholds the 2013 High Court judgment, which found that the funders were liable for the defence costs of the two defendants – Gulf Keystone Petroleum and Texas Keystone – and clarifies that the funders are liable for full indemnity costs. It also raises the cap that can apply to these indemnity costs. As a result of the ruling, the funders will be forced to pay the defendants' costs of more than £20m.

The oil companies are seeking to recoup their costs from the litigation funders because Excalibur has no assets. Excalibur's funders include the now disbanded BlackRobe Capital Partners, which was founded by John 'Sean' Coffey, who in 2013 joined New York law firm Kramer Levin Naftalis & Frankel; retired Simpson Thacher & Bartlett partner Michael Chepiga; and Timothy Scrantom of US boutique Scrantom Dulles International, which advises litigation funders.

The other funders are New York-based hedge fund Platinum Capital Partners and a UK entity called Psari Holdings, which was controlled by Greek shipping magnates Adonis and Filippos Lemos.

CC's role for Excalibur has faced scrutiny after it emerged in that the father of the firm's lead partner Panayides had been chairman of one of the Lemos family's ship management companies, while his brother was a "long standing and trusted" employee of a Lemos family company.

The Court of Appeal's ruling also reveals that CC was working for a contingency fee in which it could have recovered 140% of its usual fees, plus a discretionary success fee. This was the first time the magic circle firm had entered into a contingency arrangement, the court said.

In the ruling, Lord Justice Tomlinson states: "CC themselves had from the outset an acute conflict of interest, the extent of which worsened as their own investment in the case increased over time. It should have been obvious to any astute businessman." CC declined to comment.

Richard Waller QC of 7KBW Barristers and Richard Eschwege of Brick Court Chambers led the appeal for Texas Keystone and Gulf Keystone. Texas Keystone has also been represented by Jones Day partners Stephen Pearson and Roy Powell, while Memery Crystal is also representing Gulf Keystone.

"The Court of Appeal's ruling should serve as a warning to third-party funders of the potential perils of financing expensive international litigation without carrying out a rigorous analysis of the claim, the party to be funded and the witnesses who will support the claim," said Jones Day's Pearson.

"This will make funders more cautious about the diligence they do at the beginning and during the case. It's pretty clear now you do need to engage an independent firm to monitor the case and review it properly throughout – that may increase the care and attention funders have to take and the amount of money spent on lawyers," Pearson added. "Previously, some funders would take a 'hands off' approach once invested – now it is clear they are supposed to be monitoring and checking on the case at regular intervals."

Withers partner Chris Coffin, who advised Psari, noted that his clients did not play an active role in the underlying case.

"Our clients had no first-hand knowledge of or involvement in the conduct of the underlying litigation, as they were content to leave that to CC and counsel instructed by CC," Coffin said. He noted that the court accepted that the funders "did nothing discreditable in the sense of being morally reprehensible or even improper".

The funders put up a total of £31.75m to bankroll the case and provide security for opposing counsel's costs. Psari and affiliates of Platinum, which saw two Cayman Islands-based funds it controls plunge into bankruptcy last month, put up the most money. Platinum paid £14m, Psari paid £13.75m, and BlackRobe contributed £4m.

Under its funding agreement, Psari stood to pocket 21.6% of the recovery. In theory, Psari could have received as much as £320m, a return of around 1,450%, according to the court. BlackRobe and the other funders stood to recover up to seven times their funding plus interest.

Psari was represented on the appeal by John Wardell QC of Wilberforce Chambers. The Platinum entities were represented by Ian Croxford QC of Wilberforce. BlackRobe, which closed in 2013, did not make an appearance in the appeal. Because the defendants have joint and several liability, it is possible that the plaintiffs will pursue their recovery against the deeper pockets of Psari.

The litigation funding industry filed amicus briefs arguing that funders should not be on the hook for the full costs, claiming that it would hamper access to justice for plaintiffs with limited resources. The court rejected that argument.

"I do not myself think that commercial funders are greatly motivated by the need to promote access to justice and nor do I suggest that they should be," wrote Lord Justice Tomlinson in the judgment. "They are, as it seems to me, making an investment and are motivated by largely commercial considerations."