Source: Sharecast
Three businesses owned by the company were charged with "widespread and alarming" failures and the GC had considered suspending its licence.
Owned by online gaming and betting giant 888, William Hill allowed customers to deposit large sums of money without the companies conducting any checks, the regulator said.
These included allowing a customer to open a new account and spend £23,000 in 20 minutes, another spent £18,000 in 24 hours, while a third was able to spend £32,500 over two days, all done without any checks.
Fines will be paid by WHG (International) Limited, which runs williamhill.com - which will pay £12.5m - Mr Green Limited, which runs mrgreen.com, £3.7m; and William Hill Organisation Limited, which operates 1,344 gambling premises across Britain, £3m.
Rival Entain was handed the previous highest fine of £17m last year. The British government has been changing gambling laws to prevent addiction by capping the maximum stake on betting terminals and banning the use of credit cards by punters to place bets.
Reporting by Frank Prenesti for Sharecast.com