Major global banks including Barclays, JP Morgan, RBS, UBS, and Citigroup are being sued by investors over allegations they rigged the global foreign exchange (Forex) market.

According to US law firm Scott + Scott, the claim which is estimated to be worth more than £1 billion (US$1.24 billion), was filed at the Competition Appeal Tribunal (CAT) on Monday.

The “massive” action is a “perfect” case to be brought as a so-called opt-out collective class action for breaches of UK or European Union competition law, the Scott + Scott lawyer David Scott told Reuters.

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“It is a very difficult case to put together individual damages which are significant enough,” he said.

Michael O’Higgins, the former chairman of British watchdog the Pensions Regulator, who is leading the lawsuit, said the total value of the claim would depend on the number of Forex trades executed in London for UK-domiciled units.

Given the size of London’s Forex market, the total value would likely exceed a billion pounds, O’Higgins said.

“Even on a relatively conservative assumption it’s certainly a billion pounds and possibly several,” he said, adding that “Markets should be fair as well as free and in this case the markets weren’t fair.”

Forex manipulation was one of a series of rate-rigging scandals to engulf the global financial sector. Criminal traders at some of the world’s biggest banks conspired to manipulate the $5.4 trillion-a-day market, while making handsome profits in the process.

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They rigged the foreign currency trades via email and online chat rooms with such names as ‘The Cartel’, ‘The Mafia’, and ‘The Bandits’ Club’, to the detriment of the companies and investors who had placed trust in them. The rogue traders then relayed instances where they bolstered their profits, while lobbying their managers for bigger bonuses.

Some of the banks have already paid more than a combined $11 billion in fines to settle US, British, and European regulatory allegations that traders rigged the currency markets.

In May, the EU fined five banks a combined €1.07 billion ($1.19 billion) for Forex rigging through cartels of traders known as ‘Essex Express’ and ‘Three Way Banana Split’.

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