William Hill has been fined £6.2m by regulators over money laundering and problem gambling failures.
It is the second biggest penalty ever levied by the Gambling Commission and the largest in relation to money laundering.
The regulator said 10 customers were allowed to deposit large sums of money linked to criminal offences.
It said William Hill did not seek information about the source of the funds or establish whether they were problem gamblers.
The commission said senior management at William Hill “failed to mitigate risks and have sufficient numbers of staff to ensure their anti-money laundering and social responsibility processes were effective”.
Gambling Commission executive director Neil McArthur said: “This was a systemic failing at William Hill which went on for nearly two years and today’s penalty package reflects the seriousness of the breaches.
“Gambling businesses have a responsibility to ensure that they keep crime out of gambling and tackle problem gambling – and as part of that they must be constantly curious about where the money they are taking is coming from.”
The regulator identified failures taking place between November 2014 and August 2016.
On one occasion, a customer who was funding his gambling habit by stealing from his employer was allowed to deposit £541,000 over 14 months.
An operator had made the assumption, after a chat with the customer, that he was earning as much as £365,000 a year – when in fact he was on a salary of £30,000, the commission said.
In another instance, a customer deposited £653,000 over 18 months, during which he triggered an “amber risk” alert which should have seen his file passed to managers for review.
This did not occur due to a “systems failure” and the customer was allowed to gamble for a further six months despite continuing to activate financial alerts, the regulator said.
William Hill will pay a penalty of £5m plus return £1.2m – the amount it gained through the rule breaches – to people affected by crime linked to the breaches.
Chief executive Philip Bowcock said the company had fully cooperated with the commission and introduced “new and improved policies and increased levels of resourcing” as well as launching an independent review of its processes.
He added: “We are fully committed to operating a sustainable business that properly identifies risk and better protects customers.”
Online gambling firm 888 was handed a £7.8m penalty last year for “significant flaws” in its safeguarding of customers.