Barclays has been fined £284.4m by the Financial Conduct Authority (FCA) for failing to control business practices in its London foreign exchange operations.
The bank's "inadequate and ineffective" control over its forex floor led to the biggest fine ever handed out by the FCA.
Last year the bank opted out of a mass settlement in which six other banks agreed to pay a total of £2.6bn in fines to US authorities yet it avoided an either bigger £355.5 fine by eventually settling at stage two of the City watchdog's investigation.
It comes as part of a $2.4bn (£1.6bn) fine for its role in forex rigging, with the remainder of the bill being paid to US authorities.
Read more: Barclays to settle £2bn forex-rigging case
Georgina Philippou, the FCA's acting director of enforcement and market oversight, commented:
This is another example of a firm allowing unacceptable practices to flourish on the trading floor. Instead of addressing the obvious risks associated with its business Barclays allowed a culture to develop which put the firm’s interests ahead of those of its clients and which undermined the reputation and integrity of the UK financial system. Firms should scrutinise their own systems and cultures to ensure that they make good on their promises to deliver change.
According an FCA statement, Barclays engaged in "collusive behaviour" in which its traders and those from other banks formed tight-knit groups to manipulate fix rates and ensure it sold a particular currency to clients at a higher rate from which it bought it to ensure maximum profit for Barclays.
Members of the group referred to themselves as "the players" and "the three musketeers".