The competition regulator is to take action against some online gambling companies which it suspects of breaking consumer law.
The Competition and Markets Authority (CMA) said some punters did not get the deal they expected from sign-up promotions offering cash bonuses to attract them to gaming websites.
The CMA also said the firms were “unfairly holding onto people’s money”.
Online gambling companies should “play fair”, said the CMA.
Nisha Arora, CMA senior director for consumer enforcement said: “New customers are being enticed by tempting promotions only to find the dice are loaded against them.
“And players can find a whole host of hurdles in their way when they want to withdraw their money.”
The CMA launched its investigation into the gambling sector in October 2016. It has since heard from about 800 “unhappy” customers and has “demanded companies answer questions about how they operate, and closely examined the play on a range of websites”.
As a result it has identified “a number of operators engaging in practices likely to be breaking consumer law”, which is why it is taking enforcement action.
Initially the CMA is talking to the companies, which it says it cannot name, demanding that they change their practices.
The firms can offer undertakings about how they intend to do that. If they do not meet the requirements, the CMA can take them to court. The court could fine the companies or ultimately revoke their licences.
The online gambling sector has grown by about 150% since 2009 and is worth £4.5bn. The CMA said more than 6.5 million people regularly use the sites.