Gambling firms Ladbrokes, William Hill and PT Entertainment have agreed to change their online promotions after the competition regulator found they can "trap" players' money.
The Competition and Markets Authority (CMA) told the gambling sector to "raise its game" over concerns that customers could be made to play for longer than planned before being able to withdraw their own money.
It said Ladbrokes, William Hill and PT Entertainment had formally committed to change the way they offer bonus promotions to ensure players can always access and release their own money.
The changes must be adopted across the sector, and firms that fail to do so will face regulatory action from the Gambling Commission, the CMA said.
The changes mean that players will not need to play multiple times before they can withdraw their own money.
Firms must ensure any restrictions are made clear to players, and cannot rely on vague terms to confiscate players' money.
They also must not oblige players to take part in publicity.
The promotions are designed to attract players to casino-like gaming websites by offering bonus funds when players put in their own money.
The CMA said there was particular concern that players were being made to play for longer and therefore gamble more than they planned before they could get their own money back.
It found the problems were common across the £4.7 billion online gambling sector.
CMA project director George Lusty said: "Gambling always carries a risk, but players should never face unfair restrictions that prevent them from getting at their money.
"Firms mustn't stack the odds against players by putting unfair obstacles in their way or making it difficult for them to stop gambling when they want to.
"We welcome the commitment from these leading firms to address the problems our investigation uncovered, by making important changes to their terms and conditions.
"We now expect others to follow, and look forward to the Gambling Commission's continued work to make sure all operators in this sector play fair with their customers' money."
Gambling Commission executive director Sarah Gardner said: "We expect all Gambling Commission-licensed businesses to immediately review the promotions and sign-up deals they offer customers and take whatever steps they need to take, to the same timescales agreed by the three operators, to ensure they comply.
"Operators should be very aware that we will continue to work closely with the CMA to ensure customers are getting a fair deal across the gambling industry."
Brian Chappell, founder of campaign group Justice4Punters, said: "It's nearly 30 months to the day since Justice for Punters sent evidence of the use of unfair terms and conditions to the CMA and Gambling Commission. We're delighted they decided to work together on this investigation.
"Since that day millions have been lost and many people have been conned by the T&Cs in these promotions, but hopefully this will now stop."