The gambling regulator has handed online casino firm Genesis Global a £3.8 million fine due to money laundering and social responsibility failures.
The Gambling Commission suspended the company's licence for three months last year but lifted it after improvements by the firm.
However, it concluded on Thursday that the business - which runs 14 websites including genesiscasino.com, casinoplanet.com and casinocruise.com - will face a £3.8 million penalty.
It also warned Genesis it must undergo further extensive auditing to keep its gambling licence.
Helen Venn, executive director of the commission, Sportstoto said: "All gambling businesses should pay very close attention to this case.
"The commission will use all tools at its disposal to ensure consumer safety and that extends to stopping a business from actually operating.
An online internet gambling site.
"Failing to follow rules aimed at keeping gambling safe and crime-free will never be a viable business option for gambling businesses in Britain."
The Gambling Commission said Genesis's social responsibility failures included the business not producing any meaningful response to a customer who spent £245,000 in three months despite the business finding out after three days that it was an NHS nurse earning £30,000 a year.
The regulator also highlighted that Genesis did not produce a responsible reaction to a customer who lost £197,000 over six months.
On the day the customer closed her account, saying she wanted to spend more time with her family, she was allowed to open another account with the business and deposit £200, the commission said.
Genesis was also found to have committed a number of money-laundering failures, including allowing a customer to deposit more than £1.3 million and lose £600,000 before carrying out sufficient checks on source of funds.