The competition watchdog is to take enforcement action against online gambling companies suspected of breaking consumer law.
The Competition and Markets Authority (CMA) said that it is taking the action because punters are not getting the deal they expect from sign-up promotions, and some operators are "unfairly holding on to people's money".
Sign-up promotions are designed to attract players onto casino-like gaming websites by offering bonus cash when they put in their own money.
The move follows a joint programme with the Gambling Commission and comes amid concerns that promotions offered by some online bookies come with terms and conditions that are often "confusing and unclear and, in some cases, may be unfair".
The CMA said: "Customers might have to play hundreds of times before they are allowed to withdraw any money, so 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 watchdog added that, having identified a number of operators engaging in practices likely to be breaking consumer law, it is now taking enforcement action and has a range of powers at its disposal to bring any illegal activities to an end.
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."