Scandal-hit Barclays is reportedly under investigation over allegations it lent Qatar money to invest in the bank in 2008 as part of an emergency cash call to avoid a Government bailout.
The probe comes as the terms of the bank's fundraising at the height of the financial crisis are already being scrutinised by the Financial Services Authority (FSA) and the Serious Fraud Office.
Former Citi banker Peter Hahn, now at Cass Business School, told the Financial Times: "The concept of lending money to any investor to purchase your own shares raises a series of immediate questions about disclosure and other regulatory issues."
The allegation piles yet more pressure on Barclays chief executive Antony Jenkins as he battles to repair the bank's reputation following a string of scandals, including the Libor rate rigging affair and mis-selling of payment protection insurance.
Barclays received cash injections in 2008 worth a total of £6.1 billion from Qatar Holding, which is a subsidiary of the Qatar Investment Authority, and Challenger, an investment vehicle of Sheikh Hamad bin Jassim bin Jabr al-Thani, the prime minister of Qatar and his family.
Investors from Abu Dhabi and other sovereign wealth funds also pumped cash into the group as part of a capital raising to prevent the Government bailing it out - a move that helped it avoid the fate suffered by part-nationalised Royal Bank of Scotland and Lloyds Banking Group.
But existing shareholders complained the terms offered to the new investors were too attractive, while the fees paid for the deal are also thought to be under investigation.
Barclays and the FSA were not immediately available for comment.