Paddy Power Betfair has agreed to pay £2m for failing to step in quickly enough when customers appeared to be problem gambling.
The Gambling Commission identified instances where the bookie did not intervene soon enough to customers engaging in harmful gambling behaviour, including when one customer staked £86,000 in just over two weeks but "no manual review of the account took place".
Betting firms are required to have systems and processes that monitor concerning gambling activity.
Flutter Entertainment, which owns Paddy Power Betfair, said it takes customer safety seriously and believes the issues highlighted by the watchdog will not be repeated.
The Gambling Commission said a compliance assessment last year uncovered social responsibility failures at the firm where customer interactions "fell far short" of what is required.
