Graham Calvert is suing William Hill for £2m
Greyhound trainer Graham Calvert, 28, from Houghton-le-Spring, near Sunderland, claims the company failed in their "duty of care".
He claims he was allowed to place bets after asking the company to close his account under a self-exclusion scheme.
William Hill denies any wrongdoing and says it cannot be held legally liable for Mr Calvert's losses.
At the High Court the firm was accused of manipulating his gambling disorder to gain as much revenue as possible.
Anneliese Day, representing Mr Calvert, told Mr Justice Briggs on Wednesday that William Hill should be held liable because it failed to operate its own self-exclusion policy.
She told the judge: "What in fact occurred was that William Hill actively monitored and manipulated the claimant's gambling disorder in order to gain as much revenue for their business as possible."
She said Mr Calvert was hoping to establish in law for the first time that bookies do owe a duty of care in his circumstances.
Ms Day said William Hill "negligently sought" to encourage Mr Calvert to go on betting sprees of hundreds of thousands of pounds at a time.
She also said the scale of her client's gambling was "staggering". He had periods of mania when he placed huge multiple bets in the space of a few hours, she added.
He lost around £347,000 in one bet alone when he backed the US to win the 2006 Ryder Cup.
It is alleged William Hill allowed Mr Calvert to open two new accounts and to make bets totalling around £3.5m between June and December 2006.
During this period he lost a total of £2.1m.
Ms Day said she would be calling psychiatrists to give evidence during the five-day hearing to describe how Mr Calvert had become a "pathological gambler" which is recognised as a mental disorder.
The court heard that when other bookmakers refused his bets, Mr Calvert went back to William Hill, which allowed him to stake huge sums, even opening up a branch so he could place a £100,000 cash bet.
Ms Day added: "It is also clear from William Hill's own evidence and records that they were well aware that the claimant was continuing to bet in cash in an uncontrolled and destructive manner.
"Indeed his behaviour was being closely monitored as there were concerns as to where someone such as Mr Calvert was obtaining such large amounts of money from."
The hearing continues.