Canary Wharf
Several banks that face potential litigation in the UK for fixing foreign-exchange rates are based in Canary WharfReuters

Some of the world's biggest banks will face fresh legal challenges in the UK this autumn over allegedly cheating their clients on foreign currencies that could cost them billions.

Twelve banks, including HSBC, Royal Bank of Scotland, Barclays and US banks JPMorgan and Citigroup have paid out £6.5bn (€9.1bn, $10.1bn) to date for fixing foreign-exchange rates.

"Regulators around the world have been investigating banks to see whether they were manipulating forex rates," said a spokesman for Scott and Scott, the US legal firm that plans to bring new cases against the banks this autumn. "Every regulator has found them to be guilty. Once the guilt is established, it's hard to dispute."

In the US on 20 May, Citigroup paid $394m to settle a private lawsuit brought against it by investors who said it was manipulating foreign-exchange rates.

That was just part of a $1.66bn package of fines and settlements the bank dished out as it faced litigation on several fronts — including from the US Justice Department.

Early in 2015, Bank of America paid out an $180m settlement, while UBS and JPMorgan settled in multimillion dollar agreements in the same antitrust class action.

These private settlements followed £2.6bn in fines paid to UK regulators by six banks in the same matter in November 2014.

The UK's financial regulator, the Financial Conduct Authority, "has already taken regulatory action against firms, as have other regulators around the world", said a spokesperson at the FCA.

Now "we would be bringing claims on behalf of institutions and investors" in Europe said Scott and Scott's spokesman. The cases would be litigated in the UK.

The firm's managing partner David Scott is on a train to Paris where he will be meeting with potential claimants and then heading to Berlin to do the same. Scott has been in Europe on and off over the past year speaking with multinational businesses, pension funds, and central banks.

"It is very safe to say that the damages suffered by the clients are in the tens of billions of dollars," Scott told the BBC in an interview on 14 July.

"Imagine a building collapses, you would say the building wasn't built properly and those that owned it might go after those that built it — those are the regulators," said Scott and Scott's representative. "These clients are the poor people stuck underneath the building."

In investigations, regulators from the UK and US found FX traders met in online chat rooms using names such as "the Cartel," "the 3 musketeers," and "the players". They were found to have colluded to fix rates for currencies that they sold to investors. By doing this they made the bank bigger revenues and bonuses for themselves.

Scott and Scott has developed software tools that allows the firm to quantify the losses their clients face by crunching two to three years worth of millisecond trading data.

"This is not an easy thing to do," said the Scott and Scott spokesperson. "We are not explicitly saying who it will be. But if you see we've brought claims against 12 banks in the states, it's pretty likely we will bring a case against the same."