Online gambling company Mr Green has been slapped with a £3 million penalty for failing to protect gambling addicts.
It also failed to crack down on customers using its site for money laundering, the Gambling Commission said.
The 13-year-old company, which is owned by betting giant William Hill, failed to freeze the account of a player who immediately gambled away a £50,000 win and deposited thousands more.
It also accepted a 10-year-old document of a £176,000 claims payout as satisfactory proof that a customer had enough cash to continue to play.
Another player was allowed to use a photograph of a laptop screen showing a sum in dollars on a site purporting to be a crypto currency trading account as proof of adequate source of funds.
Mr Green is the ninth company to face action as part of a probe by the Gambling Commission into safeguarding failures by online casinos and ineffective measures to prevent money laundering.
The regulator has issued more than £20 million in penalties since 2018.
Twenty-two managers at these businesses have had their personal management licences examined, with six surrendering their licences and six receiving a formal warning, the commission said.
One received advice on their conduct, no further action was taken against two, while seven are still being investigated.
The money owed by Mr Green will go to the National Strategy to Reduce Gambling Harms – an initiative between charities, health services and the gambling industry to prevent problem gambling.
It is also focused on providing treatment and support to addicts.
Richard Watson, the Gambling Commission’s executive director, said: “Our investigation uncovered systemic failings in respect of both Mr Green’s social responsibility and anti-money laundering controls which affected a significant number of customers across its online casinos.
“Consumers in Britain have the right to know that there are checks and balances in place which will help keep them safe and ensure gambling is crime-free – and we will continue to crack down on operators who fail in this area.”