The European Union's ethics watchdog is quizzing the President of the European Central Bank, Mario Draghi, over his and the ECB's involvement in an exclusive forum with bankers and fund managers.
In a letter to Draghi that was published today, European Ombudsman Emily O'Reilly said the meetings of the Group of Thirty, where central bankers, economists and financiers talk behind closed doors, are "not transparent".
Ms O'Reilly questioned the ECB president's membership of the club.
"Where ECB members attend meetings organised by the Group of 30, they must abide by Treaty transparency requirements," Emily O'Reilly said in the letter. "However, Group of 30 meetings are not transparent."
Ms O'Reilly, who watches for ethics lapses at European institutions and can make non-binding recommendations, asked the ECB whether it would consider publishing agendas and summaries of those meetings.
Mario Draghi has until September to reply to the letter in writing.
The new inquiry was triggered by a complaint by activist group Corporate Europe Observatory, which says proximity between ECB officials and the G30 is incompatible with Frankfurt's role as the euro zone's top banking watchdog, taken up in 2014.
The G30 includes the chairmen of several commercial banks, such as JPMorgan's Jacob A Frenkel and UBS's Axel Weber.
Both firms have units in the euro zone that are directly supervised by the ECB.
A similar complaint was rejected by the Ombudsman of the time in 2012.
But O'Reilly decided the ECB's new supervisory duties warranted a fresh enquiry.
A spokesman for the ECB said: "We are cooperating with the Ombudsman, as we did with the previous inquiry."
"The group in question here is very diverse, and we see it as a relevant forum to engage with, always remembering that we have a range of rules and instruments in place to avoid apparent or potential conflicts of interest," he added.
In the letter, Ombudsman O'Reilly asked why Draghi's membership in the G30 was in the public interest and whether it could really be considered personal given that he was replaced by his deputy, Vitor Constancio, in one occasion.
"It may be argued, therefore, that the ECB’s involvement in the Group of 30 is rather of an institutional nature," the Ombudsman said in the letter, dated July 4th.
She also questioned whether the ECB found it appropriate that the chair of its own ethics committee, Jean-Claude Trichet, who may be called to assess the bank's role in the Group of 30, was also the honorary chairman of the same group.
Ties between the ECB and financial sector firms have been in the spotlight since 2015, when executive board member Benoit Coeure discussed the bank's money-printing plans at a private event with hedge funds.
This led to Ombudsman O'Reilly appealing to Draghi to scrap such meetings ahead of setting policy - a recommendation that the ECB adopted.
Draghi has been a member of the G30 since 2006, when he was still the governor of the Bank of Italy.
He sits alongside Bank of England Governor Mark Carney and the Bank of Japan's Haruhiko Kuroda, as well as Nobel prize winner Paul Krugman.
ECB vice president Vitor Constancio and Sabine Lautenschlaeger, who represents the ECB's supervisory arm on the board, have also spoken at G30 meetings.