Online gambling companies are to be investigated over claims that firms are making it hard for players to collect their winnings, a watchdog has said.
The Competition and Markets Authority (CMA) will probe complaints from the Gambling Commission of unfair treatment of customers, including hard-to-win promotions and blocks on payouts.
Around 5.5 million Britons regularly use online gambling websites and the sector has grown 146% since 2009, the CMA said.
Nisha Arora, the CMA's senior director for consumer enforcement, said: "Gambling inevitably involves taking a risk, but it shouldn't be a con.
"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."
The complaints could amount to breaches of consumer law and companies have been issued with notices requiring them to give evidence.
Gambling Commission chief executive Sarah Harrison said there were concerns that online terms and conditions appeared to "bamboozle" customers.
She 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."
Brian Chappell, 59, who set up campaign group Justice4Punters, welcomed the move.
He said: "It is absolutely essential because if you go and read any terms and conditions that any big corporate bookmakers are using, they are an absolute scandal."
He cited examples where bookmakers changed the odds after a bet had been won, claiming the customer had breached their conditions, so the payout was smaller.
In another case, a man was reportedly allowed to put £7,000 in his accounts over a short period of time without any additional checks being made, but, once his fortunes changed and he won several thousand more, his account was restricted. This capped the amount he was allowed to bet and did not allow him access to other promotions, Mr Chappell claimed.
He accused bookmakers of deliberately targeting players who use logic and mathematics to routinely win by either closing their accounts or refusing to pay out until certain conditions are honoured.
He said: "The whole thing has got out of control because they redefined what gambling means."