The interest rate-rigging scandal that has damaged the reputation of the banking sector looks likely to be reignited as Brussels is expected to impose multimillion-pound fines on a number of major firms for manipulating crucial benchmarks.

The action by Joaquín Almunia, the EU competition commissioner, will pile further pressure on Royal Bank of Scotland, the bailed-out bank already dealing with the fallout from a major systems meltdownthat left millions of customers without access to cash and allegations – which it denies – of mistreating its small business customers.

The 81% taxpayer-owned bank is reported to be among at least six major players in the financial markets, including Deutsche Bank in Germany and Citigroup in the US, caught up in the cartel investigation. The EU is said to be ready to impose record-breaking fines for alleged collusion for rigging key benchmark rates.

Brussels is thought to have focused on yen Libor, based on Japanese interest rates and priced out of London; Euribor, the Brussels equivalent to Libor; and the Tokyo rate known as Tibor. Almunia has been in discussions with banks for weeks and the resulting penalty is expected to surpass the record €1.5bn (£1.24bn) imposed on a cartel.

Each cartel could face combined fines of as much as €800m, although it was unclear on Tuesday nightwhat the exact penalties would be and how the sums would be divided. Penaltiesfor breaches of antitrust rules can theoretically be as much as 10% of turnover.

The fines are the latest to be levied on banks and financial firms for manipulating key benchmark rates. Five firms have already been fined by market regulators on both sides of Atlantic in an ongoing investigation into the manpulation of the rates, used to set interest rates on loans granted around the world.

Barclays was fined £290m in June 2012 in a move that led to the resignation of its chairman, Sir Marcus Agius, chief executive, Bob Diamond, and other senior managers. Other banks who have since been fined by US or UK regulators RBS, UBS of Switzerland and the Dutch bank Rabobank. The money broker Icap has also been fined and the FCA's investigations are ongoing.

The Libor investigation has sparked interest in a number of other benchmarks used to price financial products, particularly the foreign exchange markets, which are now being investigated by a number of regulators around the world, including the UK's Financial Conduct Authority.

Reuters reported that UBS had alerted the European Commission to the yen interest rate manipulation and would not be penalised. The Financial Times said Barclays would avoid a fine for Euribor rigging for similar reasons.

Between six and ten banks are reported to befacing fines, including Citigroup, which would be the first US bank to become embroiled in such high-profile penalties for manipulation of key rates.

Deutsche Bank and RBS will be penalised for rigging the benchmark eurozone interest rates known as Euribor. French bank Société Générale is also part of the group facing sanctions for alleged Euribor rigging, according to Reuters.

The news agency said HSBC and the French bank Crédit Agricole had not reached a settlement, while the FT said the US bank JPMorgan had also failed to do. They may face fines later.

Reuters said Barclays, Deutsche Bank, Société Générale, RBS, JPMorgan and Citigroup declined to comment and HSBC and Crédit Agricole were not immediately available to comment.