The UK Gambling Commission (UKGC) has handed down £4.5 million in fines as part of an ongoing campaign to raise consumer protection standards.
All four of the offending firms are online casino operators.
InTouch Games Limited (£2.2m) and Betit Operations Limited (£1.4m) copped the largest penalties, while MT Secure Trade will pay £700,000 and BestBet Limited £230,972.
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Each was found to have insufficient systems in place for dealing with fraud, money laundering and other player security issues.
The sanctions underline the Gambling Commission’s no-nonsense approach to compliance over the past 18 months.
The regulator wrote to dozens of UK gambling operators earlier this year and warned of harsher penalties for companies that failed to shape up.
“Our work will not stop here,” said Richard Watson, UKGC executive director.
“As a regulator, we will continue to set and enforce standards that the industry must comply with to protect consumers.
“We expect operators to know their customers and to ask the right questions to make sure they meet their anti-money laundering and social responsibility obligations.”
This is just the latest example of regulatory whip-cracking in the UK gambling industry.
In November last year, three operators were forced to pay nearly £14 million in fines for a series of breaches relating to money laundering and consumer protection.