An investor is alleging Barclays handed Qatar a $3bn (£2.3bn) loan so it could purchase the bank's shares.
PCP Capital Partners has filed documents containing the allegation at the High Court.
According to documents seen by Sky News, which first reported the story today, the alleged loan was made in 2008, at a time when the banking giant was seeking emergency funding, and the amount loaned almost mirrored the sum paid for the shares.
PCP Capital Partners, which was founded by Amanda Staveley, originally launched a case earlier this year.
Although PCP invested in Barclays shares on behalf of Abu Dhabi’s Sheikh Mansour bin Zayed Al Nahyan, the firm has also asserted that it was a potential investor, not just an adviser, and therefore could have benefited from the capital raising.
Litigation funding expert Therium has also signed on to back the case, and sources close to the case have told City A.M. that PCP is delighted to have the firm on board.
PCP Capital Partners declined to comment.
Barclays declined to comment further, but has previously said of the case: "We believe the claim against Barclays is misconceived and without merit and Barclays will be vigorously defending it."
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The Financial Conduct Authority has previously moved to hand the bank a £50m fine on claims Barclays failed to disclose advisory fees forked out to the investors behind its bailout.
In particular, Barclays paid over £300m to Qatari investors over the course of five years but did not pass on the details in fundraising proposals as it should have done.
The dealings have also caught the attention of the Serious Fraud Office (SFO), which launched an investigation into the matter in 2012.
The SFO has interviewed at least a dozen former members of the bank's senior management during the course of the investigation, including former chief executives Robert Diamond and John Varley, but no charges have been brought against any individuals.
Earlier this year, it was also reported Barclays had agreed to hand over a number of internal documents to aid the fraud squad in its investigation, something it had originally held off from doing, arguing the requested information contained legal advice protected by legal privilege.