The banking giant entered into a £7.3bn fundraising with Qatari and Abu Dhabi royals in 2008. Without this backing, it is possible the lender could have followed in the footsteps of Lloyds and RBS in requiring a state bailout.
Back in 2013, the Financial Conduct Authority (FCA) moved to hand the bank a £50m fine on claims Barclays failed to disclose advisory fees forked out to the investors behind its bailout. However, the watchdog's investigation was put on hold after the Serious Fraud Office (SFO) announced it would be probing the issue.
Now, the Financial Times has reported the FCA has now held a number of interviews regarding the case in recent weeks.
It is believed these latest interviews have been sparked by Barclays' decision last year to hand over a raft of internal documents linked to the cash call to the SFO. The bank had previously refused to do so because it said the paperwork was covered by legal professional privilege, the rules which keep advice between lawyers and their clients confidential.
The news comes shortly before the SFO investigation is expected to be put to bed. The fraud squad, which questioned at least a dozen members of Barclays ex-senior management including former chief execs John Varley and Bob Diamond as part of its investigation, has said it will reach a decision on what charges it may like to bring by the end of this month.
Both Barclays and the FCA declined to comment.
The fraud squad and the City watchdog are not the only people with an interest in the cash call's details, particularly the deal struck with the Qatari royals. PCP Capital Partners, which was founded by Amanda Staveley and invested on behalf of Abu Dhabi's Sheikh Mansour bin Zayed Al Nahyan, started legal proceedings worth almost £1bn against the bank in early 2016.
The firm is arguing it was a potential investor for the fundraising, not just an adviser, and therefore could have benefited from the capital raising. However, the bank has previously slammed the claims as having no merit.