Online bookmaker 888 has been fined a record £7.8 million for "serious failings" in its handling of vulnerable customers.
The action from the Gambling Commission follows the discovery of "significant flaws" in 888's social responsibility processes, which aim to protect consumers from gambling-related harm.
An investigation by the commission found that, due to a technical failure in 888's systems, more than 7,000 customers who had chosen to self-exclude were still able to access their accounts.
It meant customers were able to deposit a total of £3.5 million into their accounts and continue to gamble for more than 13 months.
The commission said 888's procedures were "not robust enough" and failed to protect potentially vulnerable customers.
The firm also failed to "recognise visible signs of problem gambling behaviour" displayed by an individual customer, which was so significant that it resulted in criminal activity, the commission said.
The customer staked more than £1.3 million, including £55,000 stolen from their employer.
The commission said that during a 13-month gambling binge, the customer placed a large number of bets, gambling on average three to four hours a day.
"The lack of interaction with the customer, given the frequency, duration and sums of money involved in the gambling, raised serious concerns about 888's safeguarding of customers at risk of gambling harm," it added.
Sarah Harrison, chief executive at the Gambling Commission, said: "Safeguarding consumers is not optional. This penalty package of just under £8 million reflects the seriousness of 888's failings to protect vulnerable customers.
"The 888 sanction package will ensure those affected don't lose out, that the operator pays the price for its failings via a sum that will go to tackling gambling-related harm, and that independent assurance will be given to see that lessons are learnt."
The £7.8 million includes repayment of the £3.5 million deposits made by self-excluded customers and compensation of £62,000 to the employer from whom money was stolen.
A further £4.25 million will be paid to a socially responsible cause to invest in measures to tackle gambling-related harm.
The commission has also ordered an independent audit of 888's processes relating to customer protection.