The announcement of the investigation follows concerns raised by the Gambling Commission regarding potential breaches of consumer law, including misleading promotions and unfair terms, that are being used to block the payouts given to players.

According to the CMA some 5.5 million Brits gamble online, growing some 149% since 2009, with users today logging onto sports betting, gaming and casino websites. However an initial CMA review of the £4bn-a-year sector highlighted what it feels are examples of potentially unfair treatment that it says requires comprehensive examination.

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

“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.”

In particular the CMA is concerned that:

• Players may not be able to withdraw what remains of their deposit, and any winnings, when they want to stop playing.
• Companies have wide discretion to cancel or alter odds after bets have been accepted.
• Players have very little time to make complaints and challenging decisions is restrictive.

The investigation could result in enforcement action against individual gaming sites, or prosecution in the courts and the CMA is asking people who use gambling sites and experienced such issues to provide them with further material.

Sarah Harrison, chief executive of the Gambling Commission, said they will work fully with the watchdog in the investigation.

“We expect the gambling industry to ensure terms and conditions are not unfair,” she said. “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.

“We approached the CMA to work with them to address issues in the gambling sector and are delighted to have agreed a joint programme of work to ensure terms are fair an transparent.”

If you have been affected, or feel you have been unfairly treated and want to get in touch with the CMA, all details of the investigation can be found here.