Citigroup's international broking business has been fined £12.5m by the UK financial watchdog over past failures to properly apply rules designed to halt suspicious trading activity.
The UK's Financial Conduct Authority said it had sanctioned Citigroup Global Markets for failing to adequately implement Market Abuse Regulation trade surveillance rules.
MAR rules were introduced in 2016 to expand the responsibilities of banks when monitoring for potential market manipulation and insider trading.
The FCA said the Citigroup arm "could not effectively monitor its trading activities" for certain types of illegal activity due to its failings.
It said there were gaps in the firm's arrangements, systems and procedures for trade surveillance until January 2018.
Citigroup Global Markets agreed to resolve the case and therefore saw the fine reduced from a potential £17.9m sanction.
Mark Steward, executive director of enforcement and market oversight at the FCA, said: "The framework for market integrity depends on the partnership between the FCA and market participants using data to detect suspicious trading.
"By not fully implementing the new provisions when required, Citigroup Global Markets did not carry its full weight in this partnership, impacting market integrity and the overall detection of market abuse."