Online gambling operators 32Red and Platinum Gaming, both part of Kindred Group, have been fined £7.1mln by the Gambling Commission.
The UK government body said the two online gambling sites have been penalised for social responsibility and anti-money laundering failures.
32Red Limited will pay £4.195mln while Platinum Gaming, which runs Unibet, will fork out £2.937mln.
Social responsibility failures by 32Red involved not identifying customers experiencing gambling-related harm, not implementing effective controls to protect problem gamblers, and not properly probing customer interactions.
For example, one customer was allowed to deposit £43,000 and lose £36,000 over the course of a week.
Platinum Gaming failed to identify potentially blocked or self-banned customers holding multiple accounts and failed to interact with customers potentially experiencing harm, the Gambling Commission said.
“These failures highlight clearly that both operators failed to interact with customers in a way which minimises the risk of them experiencing harms associated with gambling,” said Kay Roberts, executive director of the Gambling Commission.
“Ultimately, it is an example which all gambling operators should take notice of to ensure they protect their customers at all times.”
A much-awaited Gambling White Paper, highlighting areas of concern and possible regulation, is expected to be published in Easter.
Shares in Flutter were up 1.2% to 14,290p, while 888 Holdings were down 4% to 59.9p.