Online gambling firms and the owners of casino websites are "likely to be breaking consumer law" and could face court action, the competition watchdog warned.
Regulators also said gambling companies were "unfairly holding on to people's money".
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Nisha Arora, senior director for consumer enforcement at the CMA, 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 said it had "identified a number of operators engaging in practices likely to be breaking consumer law" and was talking to firms to agree remedial action. Without this, it warned, it could pursue court action.
It specifically highlighted the problem of "matched bets", where the site matches the amount the customer stakes. If £10 is bet, for example, it adds another £10 to double the stake.
"However," says the BBC, "in the terms and conditions of play it might state that the customer has to play several hundred times within a certain period of time before they can cash in their winnings."
Ultimately, by forcing people to play for an extended period, the firms are increasing the chance that the customer will lose or par any winnings.
Other issues identified when the inquiry was launched included hidden exclusion clauses that disqualified people from withdrawing their winnings and changes to odds made without the customer's knowledge.
