Paddy Power Betfair is to pay £2m for acting too slowly on concerning customer behaviours, including one player who staked £86,000 over 16 days and another who placed 300 bets in eight hours.
The Gambling Commission found social responsibility failures in a review of the firm last year and said some customer interactions "fell far short".
John Pierce, the watchdog's director of enforcement, said the £2m settlement reflected the seriousness of the case.
Betting firms must have systems and processes to monitor activity and identify worrying behaviours, but Paddy Power Betfair's were found to be lacking in some cases.
The commission said "the velocity of spend, increasing deposits, overnight gambling, and changing betting patterns did not appear to be identified by the licensees or acted upon until the next day".
The firm failed to intervene fast enough despite a customer betting £86,000 and losing £6,000 over 16 days, with a manual review only starting when the losses mounted.
It also didn't promptly identify another gambler who went on a 17-day spree, including one session of seven hours and 46 minutes in which they put up £20,000 in more than 300 bets.
Another gambler deposited £25,000 over 25 days before being interacted with, according to the commission's statement.
"Operators must ensure systems to identify and address harm work effectively and at the right time," said Mr Pierce.
"Over-reliance on automation and failure to intervene when clear harm indicators are present exposes consumers to unnecessary risk."
The commission described the £2m as "a payment in lieu of a financial penalty".
However, it said the firm had quickly instigated a plan to fix the failings and fully cooperated with the investigation.
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In 2023, the commission also fined Paddy Power and Betfair £490,000 for sending messages to the phones of vulnerable customers who had asked to be excluded.
A spokesperson for Flutter Entertainment - which owns Paddy Power Betfair - said it takes safer gambling responsibilities "incredibly seriously" and has industry-leading safeguards.
"Customer safety is our number one priority and there is no suggestion that any of the customers reviewed by the Gambling Commission experienced any harm," the spokesperson said.
"Our controls have evolved significantly and we recently introduced a next generation customer safety platform, with the vast majority of checks now happening in real-time.
"As such, we are confident that the issues highlighted by the commission in its public statement would not be repeated today."
(c) Sky News 2025: Paddy Power Betfair to pay £2m for failing to protect gamblers
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