Britain's financial regulator on Thursday fined Barclays bank Â£72 million ($109 million, 102 million euros) for failing to carry out proper checks on a vast so-called "elephant" deal for ultra-wealthy clients.
The Financial Conduct Authority said Barclays rushed through a huge Â£1.88-billion transaction in order to secure fees from a number of very rich customers.
"The clients involved were politically exposed persons and should therefore have been subject to enhanced levels of due diligence and monitoring by Barclays," the FCA added in a statement.
The watchdog stressed that it found no evidence of any financial crimes -- but the circumstances of the deal indicated a higher level of risk.
"This required Barclays to adhere to a higher level of due skill, care and diligence but Barclays failed to do this.
"In fact, Barclays applied a lower level of due diligence than its policies required for other business relationships of a lower risk profile."
The FCA said Barclays went to "unacceptable lengths to accommodate the clients", failing to ask for key information as it "did not wish to inconvenience" them.
It added: "Barclays did not follow its standard procedures, preferring instead to take on the clients as quickly as possible and thereby generated Â£52.3 million in revenue."
The transaction was the biggest of its kind ever handled by Barclays and was known as an "elephant" deal owing to the sheer size.
The fine comprises the Â£52.3 million that Barclays made in fees from the deal, plus an extra penalty of Â£19.8 million.
The transaction was arranged and completed in 2011 and 2012, just before Barclays became embroiled in a Libor rate-rigging scandal.
Barclays said in a statement that it had cooperated fully with the FCA inquiry. The lender added that it continues to apply "significant resources and training" to ensure compliance with legal and regulatory requirements.