Online gaming firm Mr Green, which is owned by William Hill, has been hit with a £3m penalty for failing to protect gambling addicts.
The Gambling Commission also said the company did not have effective procedures to check customers were using legitimate sources of money.
The commission said its investigation had uncovered "systemic failings".
It failed to freeze the account of a customer who won £50,000 and gambled it away before depositing thousands more.
The company also accepted a 10-year-old document showing a £176,000 claims payout as satisfactory evidence of source of funds for a customer who deposited more than £1m.
Mr Green is the ninth company to face penalties as part of a probe by the Gambling Commission into safeguarding failures by online casinos and poor measures to prevent money laundering.
The Gambling Commission has issued more than £20m in penalties since 2018.
The money will go to the National Strategy to Reduce Gambling Harms, which provides treatment and support for addicts.
"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," said Richard Watson, the Gambling Commission's executive director.
"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."
William Hill bought Mr Green in 2019 for £242m.