The Competition and Markets Authority (CMA) said it was investigating if companies were acting fairly, particularly through their use of promotions – such as free bets – amid allegations complex terms and conditions or mistakes were being cited for cutting winnings.

Other concerns included players being unable to withdraw deposits, and any winnings, when they want to close accounts and short time limits for submitting complaints. The regulator said it wanted punters to submit any evidence of unfair practice.

Nisha Arora, its senior director for consumer enforcement, said: “Gambling inevitably involves taking a risk, but it shouldn’t be a con.

“We’re worried players are losing out because gambling sites are making it too difficult for them to understand the terms on which they’re playing, and may not be giving them a fair deal. We are now investigating to see whether firms are breaking the law.”

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