France's top lenders are suing the European Central Bank to get an exemption from holding capital against deposits parked with a state-owned fund, the most high-profile challenge to supervision from Frankfurt to date.
As well as providing euro zone banks with funding, the ECB has been their main regulator for the past two years, tasked with ending cozy relationships between the industry and national authorities that contributed to the financial crisis.
The Frankfurt-based institution has been sued repeatedly over its bond-buying programs and by smaller banks seeking to escape its supervision.
But this is the first case brought by major banks in the euro zone and is a rare confrontation between France's financial elite and the ECB's supervisory board, led by the former head of France's own banking regulator, Daniele Nouy.
The lawsuits have been brought by BNP Paribas, Societe Generale, Credit Agricole, Credit Mutuel, Groupe BPCE and La Banque Postale over the past few weeks, filings with the European Court of Justice show.
Sources with direct knowledge of the cases told Reuters the banks are protesting the ECB's demand that they set aside capital against special deposits they have with state investment institution Caisse des Dépôts et Consignations (CDC).
The legal action comes amid heightened tension between banks and the ECB, which is inundating the financial sector with excess cash to try to stimulate growth while charging banks for depositing it with the central bank overnight.
"You are seeing banks more and more go to court to challenge the supervisor," a senior legal source said. "Years ago that was unthinkable."
The ECB and the banks declined to comment.
Tax-free savings accounts in France, such as the Livret A, are widely used by smaller retail savers and they were worth some 400 billion euros ($417 billion) at the end of last year.
Slightly more than half that money was then placed by the banks with the Caisse des Dépôts et Consignation, which uses the funds to invest in public housing and other projects.
Banks argue that this exposure to the CDC should not count towards the calculation of their leverage ratio, which is a measure of a lender's capital as a proportion of total assets.
They cite a European Commission act saying regulators may grant an exemption for, "deposits that the institution is legally obliged to transfer to (a) public sector entity ... for the purposes of funding general interest investments".
Under EU rules due to come into full force in 2018, banks must hold capital worth at least 3 percent of their total assets. By stripping out the deposits with the CDC, banks would get a higher leverage ratio for a given amount of capital.
For example, La Banque Postale said its capital was worth 5 percent of assets at the end of June but if its deposits with the CDC were included in the calculation, its leverage ratio would fall to 3.4 percent.
A source close to the ECJ said a decision on the suits may take about two years, meaning it was unlikely to come before the leverage ratio rule is phased in.
La Banque Postale and BPCE, which long had a monopoly on French regulated savings accounts, would be worst hit.
"It's obviously important for Banque Postale," Olivia Perney Guillot, who rates the bank for Fitch said. "The ratings already take into account the potential lifting of the national exemption."
BPCE and Credit Agricole knocked 30 and 15 basis points respectively off their leverage ratios earlier this year to take account of their deposits with the CDC.
Their complaints should be familiar to Daniele Nouy, who worked at the French central bank before becoming the head of France's prudential authority in 2010.
In 2013, before moving to the ECB, she warned that French banks would be "strongly impacted" by new liquidity rules due to their exposure to regulated savings accounts, among other reasons.