Jackpotjoy has told Gambling Insider it was "deeply sympathetic" towards a customer who lost £633,000 ($765,690) over a six-year period.
A customer, whose identity has been protected, told UK documentary "BBC Panorama: Addicted to Gambling" Jackpotjoy was continually offering free cash bets to continue gambling with the operator.
The customer also said Jackpotjoy issued a £1,000 cash bonus after finding out her father had passed away, as well as calling and e-mailing when the customer had gone a day without betting.
Jackpotjoy has not denied issuing the customer with cash bonuses and rewards.
BBC Panorama obtained figures from the Gambling Commission, which revealed a total of 8,266 complaints were made last year to British betting firms.
This represents over a 5,000% increase compared to 2013, when there were 169.
A Jackpotjoy spokesperson told Gambling Insider: "We have informed Panorama we are deeply sympathetic to the unfortunate personal circumstances experienced by the customer during their playing period with us.
"As a loyal customer, we had frequent and personal engagement with her over the course of six years, which involved advising on and encouraging the use of our responsible gambling tools.
"We have always acted in accordance with the relevant regulatory requirements, as we understood them to be at the time."
Marc Etches, CEO of GambleAware, responded to the story with the following statement: "Gambling companies have a clear responsibility to make sure their customers are able to gamble in a safe environment, wherever they are.
"We urge all operators to make people aware of the risks of gambling and clearly signpost to the free help and advice that is available at BeGambleAware.org."