The owner of Ladbrokes Coral has been fined £5.9m for not protecting vulnerable customers and for failings in its anti-money laundering measures.
The Gambling Commission says that over a three-year period, Ladbrokes and Coral failed to put effective safeguards in place to "prevent consumers suffering gambling harm".
One customer lost £98,000 and had asked the company to stop sending promotions.
But the firm failed to carry out "social responsibility interactions".
The Gambling Commission said the problems occurred between November 2014 and October 2017, after which GVC Holdings bought Ladbrokes Coral in March 2018.
GVC Holdings will pay £4.8m and divest £1.1m "gained from customers as a result of its failings".
In one instance, a Ladbrokes customer had 460 attempted deposits into their gambling account declined. However, they were still able to lose £98,000 over two and a half years.
The commission also highlighted a Coral customer who spent £1.5m over nearly three years, during which time they logged onto their account an average 10 times a day for one month and lost £64,000 in one four week period.
It said Coral "did not ask the customer to evidence their source of funds and could not provide evidence of any social responsibility interactions being carried out".
Richard Watson, executive director of the Gambling Commission, said: "These were systemic failings at a large operator which resulted in consumers being harmed and stolen money flowing though the business and this is unacceptable."
GVC said it "acknowledges and regrets" that certain legacy systems and processes in place at Ladbrokes and Coral "did not adequately meet the regulatory requirements".
"These historical failings were unacceptable and since the acquisition, I have overseen a systematic review of the enlarged group's player protection procedures and the individuals responsible for these problems have exited the business," added GVC chief executive Kenneth Alexander.
"I am confident that we now have in place a robust and industry-leading approach to player protection."
Shares in GVC Holdings rose 0.59% to 611.37p.
As well as the Ladbrokes and Coral brands GVC also owns gambling outlets bwin, Crystalbet, Eurobet, Neds and Sportingbet.
Its games brands include CasinoClub, Foxy Bingo, Gala, Gioco Digitale, partypoker and PartyCasino.
The penalty for Ladbrokes Coral Group is one of the biggest imposed by the gambling watchdog.
UK gambling firm 888 had to pay a record £7.8m in August 2017 as a result of serious failings in its handling of vulnerable customers.
Online gambling business Daub Alderney received a £7.1m penalty in November 2018 for failing to follow rules aimed at preventing money laundering and protecting vulnerable consumers.
William Hill had to pay around £6m for systemic senior management failure to protect consumers and prevent money laundering in a penalty package in February 2018