The record fine for online bookmaker 888 comes amid concern from campaigners who say they are aware of "numerous" failures in protecting vulnerable customers.
The Gambling Commission found that more than 7,000 customers who had chosen to self-exclude - when a punter asks to be excluded from gambling for a set length of time - were still able to access their accounts due to a technical failure in 888's system.
The commission said the failure was an isolated incident.
Brian Chappell, founder of campaign group Justice4Punters, said: "It's really positive to see the regulator taking further action on protecting vulnerable people."
But he added: "We receive numerous cases where people who've self-excluded are allowed to gamble again.
"The period of time it takes companies to pick this up varies immensely.
"This is unacceptable and the whole industry needs to tighten up its policies. We would see more fines being inevitable without fast change."
The commission found that 888 customers who had self-excluded deposited a total of £3.5 million into their accounts and continued to gamble for more than 13 months.
In one separate case reported to Justice4Punters concerning another company, a customer who had been self-excluded with most bookies since 2013 lost thousands of pounds over a period of around three weeks, and "at no stage" received the mandatory Responsible Gaming reality check email or phone call.
In an email to the campaign group, the man said: "In total they let me deposit over 25K without any alarms being raised or checks, and when they did finally contact me it was to tell me I was now a VIP customer with VIP personal manager."
Mr Chappell said: "Our caseload strongly suggests that this example is not unique and more fines would seem likely without fast change throughout the industry.
"Interestingly, our experience outlines that when gamblers win as opposed to losing, the 'Know your Customer' processes are very different.
"People are frequently asked for additional ID, including copies of utility bills, passport photo pages, even proof of income, before winnings are paid."