Brussels is poised to levy multibillion-euro fines on banks that took part in a cartel to rig two key global interest rate benchmarks as the cost of financial misconduct continues to spiral.

Royal Bank of Scotland, Deutsche Bank and Société Générale are set to pay the fines next month to settle allegations that they acted in concert to manipulate either Euribor, Yen Libor or both benchmarks, according to people familiar with the talks.

A further three banks – JPMorgan, HSBC and Crédit Agricole – have been involved in the talks but are holding out from signing a joint Euribor settlement, the people said. These banks are likely to face formal charges that could lead to fines at a later stage.

The news comes at a torrid time for the world’s biggest banks, which are facing a litany of scandals ranging from allegations that they manipulated the foreign exchange market to the mis-selling of mortgage securities and consumer products.

Next month’s bumper EU settlement relates to alleged collusion between interest rate traders in two separate cartels, which sought to gain financially by manipulating the benchmarks.

The commission has warned some banks that it is seeking a fine of as much as €800m each for groups involved in both cartels. The precise level will depend on a mix of culpability and market share.

Even with three banks holding-out, the settlement is expected to smash antitrust records in Europe; to date the highest combined EU fine for a cartel is about €1.5bn. If the commission issues fines at the top of the range for all six banks, the aggregate could hit €5bn, though people involved in the talks said the final number could be significantly lower.

The commission’s Euribor case relies in large part on evidence submitted by Barclays, which has secured immunity from EU cartel fines relating to Euribor.

Four banks – Barclays, RBS, UBS, Rabobank – and broker ICAP have settled with regulators around the world over wider Libor abuses, paying a combined $3.5bn in fines. However, the EU antitrust authorities have not been party to those deals, preferring instead to seek a multi-bank settlement of their own.