The Gambling Commission has fined four more betting firms for failing to prevent money laundering and effectively protect problem gamblers.
In November the watchdog fined companies, including Casumo, Videoslots and Daub Alderney, a total of £14m.
InTouch Games, Betit Operations, BestBet and MT Secure Trade now face fines totalling £4.5m for similar failings.
The financial penalties are part of an ongoing investigation by the Gambling Commission, which has assessed 123 online companies.
InTouch Games Limited will pay £2.2m, Betit Operations Limited will pay £1.4m, MT Secure Trade will pay £700,000 and BestBet will pay £230,972.
Since the investigation began, five companies, including CZ Holdings, have surrendered their licence to offer a betting service in the UK.
Executive Director of the Gambling Commission Richard Watson said: “We have been working hard to raise standards in the online industry to ensure that gambling is crime-free”.
“But our work will not stop here. As a regulator, we will continue to set and enforce standards that the industry must comply with to protect consumers.
“We expect operators to know their customers and to ask the right questions to make sure they meet their anti-money laundering and social responsibility obligations.”
Figures from the National Gambling Helpline estimate that there are around 30,000 problem gamblers in the UK.