Britain's FCA has fined broker ED&F Man Capital Markets Ltd (MCML) 17.2 million pounds ($21.35 million), the largest fine to date in connection with "cum-ex" scams in Europe.

cum-ex trading schemes, which boomed after the 2008 global financial crisis, involve banks and investors quickly trading shares around dividend date, blurring stock ownership and allowing multiple parties to claim tax refunds.

The FCA said on Monday it had fined the broker for "gross negligence" for profiting from trading strategies designed to allow clients to illegally recover taxes from Danish authorities.

The regulator said the company did not contest the FCA's findings and agreed to settle, resulting in a 30% reduction in the applicable fine.

A spokesman for MCML said the fine was related to a legacy business that closed in 2015.

The fine comes after three smaller cases of similar lapses against brokers.

So far, the enforcement activities of UK regulators related to the scheme have been dwarfed by the massive investigations led by Germany and Denmark. The investigations involve about 1,500 suspects and 100 banks across four continents, officials said.

The FCA said that between 2012 and 2015, MCML's customers illegally declared around £20 million in tax to Danish authorities, generating more than £5 million in fees for the company.

Last year, some of ED&F Man Capital Markets' assets were acquired by Marex, a financial services company.

A spokesman for MCML said the areas of the business affected by the FCA's action had been explicitly excluded from the sale and were contingent liabilities that were ringfenced as part of the deal.

"The final steps to deregulate and shut down MCML operations can now be taken," the spokesman added.