The UK Gambling Commission has handed a £19.2m fine to the William Hill betting group after it failed to protect consumers and stop money laundering.
Today' s fine was the biggest penalty ever issued by the regulator.
The regulator said the issues at the three companies in the William Hill group, itself owned by online gaming and betting operator 888, were so "widespread and alarming" that it seriously considered suspending the firm's licence.
But Gambling Commission Chief Executive Andrew Rhodes said the company was allowed to continue to operate after it recognised its failings and worked to implement improvements quickly.
"We found serious non-compliance issues around safer gambling measures, and also anti-money laundering control failings across the company," Mr Rhodes said.
Problematic practises in Britain's multi-billion pound gambling industry have forced the regulator to issue several large fines over the last year and a half. The previous biggest fine of £17m was handed to Entain last year.
The UK government has sought to tighten gambling rules in recent years to prevent addiction by capping the maximum stake on terminals and banning the use of credit cards to place bets.
At the William Hill companies, multiple failures were found by the Gambling Commission. One customer spent £23,000 in 20 minutes without any checks, for example, another lost £14,900 in 70 minutes.
Additionally, checks were not made on large sums of money deposited with the William Hill companies.
888, which completed its takeover of William Hill and related companies last year, said the problems which led to the fines related to the period before its ownership.
"After William Hill was acquired, the company quickly addressed the identified issues with the implementation of a rigorous action plan," an 888 spokesperson said.
