Some bookmakers are facing action over "unfair" sign-up promotions as part of a government crackdown.
The Competition and Markets Authority (CMA) believes customers "aren't getting the deal they expect" from some new customer bonuses.
The CMA has not named which operators are being faced with the "enforcement action".
Having identified a number of operators engaging in practices likely to be breaking consumer law, the CMA is now taking enforcement action and has a range of powers at its disposal to bring any illegal activities to an end.
The CMA is particularly concerned with wagering requirements on casino offers - where customers have to play through their bonus multiple times before they can withdraw.
The CMA said: "They don’t have the choice to quit while they’re ahead and walk away with their winnings when they want to.
"Even when players haven’t signed up for a promotion, there are concerns that some operators are stopping customers taking money out of their accounts.
"The CMA says it has been told by customers that some firms have minimum withdrawal amounts far bigger than the original deposit, or place hurdles in the way of them withdrawing their money."
Firms 'must play fair'
Nisha Arora, CMA senior director for consumer enforcement, said: "We know online gambling is always going to be risky, but firms must also play fair. People should get the deal they’re expecting if they sign up to a promotion, and be able to walk away with their money when they want to.
"Sadly, we have heard this isn’t always the case. 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.
"That’s why we are today launching enforcement action where we think the law has been broken. We are also asking people who have had difficulties withdrawing their money when they’ve gambled online to tell us about it, and help probe this issue even further."
'Too many strings attached'
Gambling Commission chief executive Sarah Harrison added: Gambling operators must treat customers fairly – but some have been relying on terms that are unclear with too many strings attached.
"While the CMA takes enforcement action on how consumer legislation is followed, the gambling industry should be under no illusion that if they don’t comply with consumer law, we will see this as a breach of their operating licence, and take decisive action."