Online gambling firms may be breaking the law by making it very difficult for players to collect their winnings, the competition regulator has said.
The Competition and Markets Authority (CMA) said gambling "shouldn't be a con" and is launching an inquiry into whether customers are treated fairly.
Online gambling has grown dramatically, and 5.5 million Britons now regularly log on to betting sites.
The industry said it would co-operate fully with the CMA's investigation.
The competition watchdog said many people found it hard to win the money they are expecting.
"Gambling inevitably involves taking a risk, but it shouldn't be a con," said Nisha Arora, the CMA's senior director for consumer enforcement.
"We've heard worrying complaints suggesting people may be lured into signing up for promotions with little chance of winning because of unfair and complex conditions."
Sarah Harrison, the chief executive of the Gambling Commission, which will work alongside the CMA on the inquiry, said: "Gambling, by its very nature, is always going to involve risk, but customers must have faith that if they win, they will not end up feeling that the deck is stacked against them because of an obscure condition that they did not properly understand."
The investigation could result in enforcement action against individual gaming sites, or prosecution in the courts.
Hard to challenge
Online gambling firms typically advertise welcome bonuses of up to several thousand pounds, or supposedly free bets.
But the small print may disqualify certain games, or require customers to spend large amounts of money before they qualify.
The CMA is also concerned that:
- Consumers can find it difficult to withdraw their deposit when they want to stop playing
- It is difficult to challenge any decisions made by the betting site
- Complaints sometimes have to be made within 7 days
- Betting sites can alter the odds without the knowledge of gamers
However, the Remote Gambling Association - which represents the industry - said there was no reason to believe there were widespread failings amongst its members.
It said it would be wrong to pre-judge the outcome of the CMA's inquiry.
I 'won' £35,000
Chris Sattin from Gloucester was playing roulette on a website called Maria Casino and won £35,000, but he wasn't allowed to withdraw his winnings.
He told Radio 4's You and Yours: "I was shaking, my adrenaline was pumping. I pressed on the iPhone to withdraw, but nothing was happening. Because I'd never won these sums of money before, I thought maybe it's only happening because it's a large sum of money and I need to contact customer services."
Maria Casino told Chris he had an account with its sister company Unibet, and he had used a self-exclusion feature on the site - something introduced by the Gambling Commission to help problem gamblers.
Chris told the company he had self-excluded only to close his account. But Maria Casino said this breached the company's terms and conditions.
You and Yours contacted Maria Casino about Chris's case and they decided to pay him the £35,000 winnings.