The Financial Services Authority has fined insurance intermediary Willis Ltd £6.9m for anti-bribery and corruption systems and controls failings.
This is the biggest fine the FSA has ever imposed in relation to financial crime systems and controls.
The regulator said Willis' failings created "unacceptable risk" payments made to overseas third parties could be used for corrupt purposes.
Willis is a global insurance broker and risk management consultant headquartered in London, with 400 offices in 120 countries and 17,000 employees.
Between January 2005 and December 2009, Willis made payments totaling £27m to overseas third parties who assisted it in winning and retaining business from overseas clients, particularly in high risk jurisdictions.
Willis failed to establish and record adequate commercial rationale to support its payments to overseas third parties, the FSA said.
It also failed to carry out adequate due diligence on overseas third parties to evaluate the risk in doing business with them, and it did not properly review its relationships with those overseas third parties, the regulator said.
Failing to do this, the FSA said, gave rise to a high risk the payments could be used for corrupt purposes such as paying bribes.
Brendan McManus, chief executive of Willis, said: "We recognise the importance of such measures in assuring ourselves and stakeholders that the risk of wrong-doing is designed out of the way we do business.
"When we discovered some of our businesses had not got that right in the past, we were swift to engage with the FSA towards today's regulatory resolution.
Tracey McDermott, acting director of enforcement and financial crime at the FSA, said: "The involvement of UK financial institutions in potentially corrupt practices overseas undermines the integrity of the UK financial services sector.
"The action we have taken against Willis shows we believe it is vital for firms not only to put in place appropriate anti-bribery and corruption systems and controls, but also to ensure that those systems and controls are adequately implemented and monitored."
Due to cooperation with the FSA, Willis qualified for a 30% discount on its fine. Had it not done so, the total fine would have been £9.85m.
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