Amanda Staveley's investment vehicle PCP Capital Partners is suing Barclays over its emergency £5.8bn fundraising in 2008.
The deal-maker is said to be seeking almost £1bn from the bank.
PCP invested on behalf of Abu Dhabi’s Sheikh Mansour bin Zayed Al Nahyan, who has since offloaded much of his stake in the bank through a hedging deal with Japanese bank Nomura, in October 2008.
The firm is reportedly alleging that it was a potential investor and not just an adviser during the deal and therefore could have benefited from the capital raising.
A Barclays spokesperson said: “We believe the claim against Barclays is misconceived and without merit and Barclays will be vigorously defending it.”
The lender is already facing a probe by the Serious Fraud Office over its dealings with Qatar during the cash call, and was handed a £50m fine by the Financial Conduct Authority over alleged misbehaviour during the fundraising, although Barclays disputed the penalty.
PCP is bringing the case against Barclays in London, according to court documents seen by the Financial Times, which first reported the story.