Secondary ticketing websites may be breaking the law, by failing to tell consumers about restrictions on the tickets they're buying.
After a year-long investigation, the Competition and Markets Authority (CMA) found widespread concerns about such websites.
It said some of them had taken action to improve, but others had not.
The CMA has not named the websites involved, but it is threatening to do so if they don't comply.
It may also take action in the courts.
Secondary ticketing websites are designed to re-sell tickets to concerts, shows and events where the original buyer can no longer attend.
The four largest sites, Get Me In, Viagogo, Stubhub and Seatwave were all investigated as part of the enquiry.
All four agreed to make some changes in March 2015, before the CMA investigation began.
"Our investigation has identified concerns that the law protecting consumers is being broken," said Andrea Coscelli, the chief executive of the CMA.
"Thousands of people use these sites and they have a right to know if there is a risk that they will be turned away at the door, who they've bought their ticket from or exactly what seat at the venue they're getting for their money."
The CMA says it must be made clear to consumers:
- whether there are restrictions on using a re-sold ticket which could stop someone getting into an event
- whom they are buying from. In some cases the seller could be a business, not an individual
- exactly where in the venue they will be sitting
The CMA has also found some additional concerns, such as "pressure selling".
In some cases websites are making false claims about the popularity of tickets, putting pressure on consumers to buy quickly.
In other cases consumers can have difficulty getting their money back, despite guarantees on the website.
The CMA also highlighted instances of speculative selling – where businesses advertise tickets they do not yet own – and cases where secondary sites are being used as primary sellers.
Several high-profile bands, including the Arctic Monkeys and One Direction, have called for tighter enforcement action on such sites.
Under the Consumer Rights Act, offending sites face fines of up to £5,000.