British authorities are looking into an allegation that Barclays loaned Qatar money to invest in the bank as part of its rescue fundraising at the height of the 2008 financial crisis, the Financial Times reported.
Qatar Holding is not accused of any wrongdoing, the FT said.
The Financial Services Authority (FSA) and Serious Fraud Office (SFO) have been looking into Barclays' emergency fundraising since July.
Qatar Holding invested £5.3bn ($8.4bn) in Barclays in June and October 2008, helping it avoid being bailed out by the government, unlike rivals Lloyds Banking Group and Royal Bank of Scotland.
Allegations over a loan to the Qataris is a new thread of the investigation, the FT said, citing two sources familiar with the situation.
A Barclays spokeswoman said: "Both the FSA and SFO investigations are on-going and, as such, we are unable to comment further."
The FSA and SFO both declined to comment.
New Barclays chief executive Antony Jenkins has been trying to restore the bank's reputation after a string of scandals, including a $450m fine in June for rigging Libor interest rates that prompted the resignations of previous CEO Bob Diamond and chairman Marcus Agius.
The deal with Qatar was controversial from the outset. Shareholders were angry Qatar was offered more attractive terms to invest than existing investors, and a sale of warrants in November left Qatar sitting on a gain of £1.7bn from its investment, according to Reuters estimates.
Qatar Holding, part of Qatar Investment Authority, which was set up by the state in 2005 to diversify investments away from oil and gas into new assets, is the bank's biggest shareholder with a 6.7 percent stake.
Barclays said in August that Britain's fraud prosecutors had launched a criminal probe into payments between the bank and Qatar, a month after revealing the FSA's investigation into dealings between the two parties.
It said the FSA probe was into the bank and four current and former senior employees, including finance director Chris Lucas.