Online gambling firms may have crossed a line by effectively cheating punters out of winnings they expected to receive, according to a regulator.

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.

Horses throw up spoil from the course as they round the bend at Flemington racecourse in Melbourne, Australia
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Campaigners claim the industry is targeting 'logic' punters
"Around 5.5 million Britons gamble online and they must be treated fairly. We've heard worrying complaints suggesting people may be lured into signing up for promotions with little chance of winning because of unfair and complex conditions.

"We're now working closely with the Gambling Commission to examine this more closely."

Gambling Commission boss Sarah Harrison added: "We expect the gambling industry to ensure terms and conditions are not unfair.

"However, operators are still not doing enough. I continue to have concerns that many of these appear to bamboozle rather than help the customer make informed choices."

A campaign group, Justice4Punters, welcomed the decision to investigate.

It claimed bookmakers were deliberately targeting players who use logic and mathematics to routinely win by either closing their accounts or refusing to pay out until certain conditions are honoured.

The group complained that such people were often restricted to low value bets - sometimes on money laundering grounds.

The Remote Gambling Association, which represents firms, pledged full co-operation with the CMA but said it did not believe there were widespread failings, as the watchdog claims.

It told Sky News: "If there are faults it is right that the CMA shines a light on them and that we collectively learn lessons from that."

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