Three online casino companies will pay penalties of £14m and one has been prevented from operating in the UK following a crackdown by the industry regulator.
The Gambling Commission said the three firms did not have "effective safeguards" to prevent money laundering and harm to customers from gambling.
Daub Alderney was fined £7.1m, Casumo £5.85m. Videoslots is to pay £1m in lieu of a financial penalty.
A fourth firm, CZ Holdings, surrendered its licence to operate in the UK.
"I hope today's announcement will make all online casino operators sit up and pay attention, as our investigations found that a large number of operators and their senior management were not meeting their obligations," said Neil McArthur, the chief executive of the Gambling Commission.
"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."
After a review which started in February, the Commission found that Videoslots had been making initial inquiries into the source of customer funds, but failed to continue to make checks.
In one case a Videoslots customer failed automated identity checks and instead provided a fraudulent driving licence, which was not detected.
The customer registered multiple fraudulent credit cards, which were also not detected. Those cards were used to deposit and gamble with "large amounts".
The customer had deposited £17,405 by the time the company's systems flagged the account. The Commission said those deposits were "suspected to be the proceeds of crime".
Videoslots accepted that it had weaknesses in its systems and had taken action to address the issues, the Commission said.
The company's settlement payment will go towards research and the treatment of problem gambling.
A review of Casumo's business, which began in January, found that the company had failed to check customers properly and monitor their activities.
Its investigation reviewed the accounts of three customers that showed potential signs of problem gambling, but did not trigger any action from Casumo.
Since then the company has taken action to improve its procedures, the Commission said.
Earlier this month, the Commission announced it was fining Daub Alderney £7.1m.
The company's monitoring of customers was also found to be inadequate, but it has since tightened its procedures.