Secondary ticketing websites are facing enforcement action following a year-long investigation by the competition regulator.
The Competition and Markets Authority (CMA) said it suspected “a number” of operators were breaking consumer law but, again, refused to identify them at this stage.
The watchdog said it was raising its concerns with those companies – requiring them to take action or face the possibility of prosecution.
The investigation initially focused on the information they provide customers.
But the CMA said it had also found evidence of pressure-selling, when websites make false claims aboutthe popularity of tickets, and difficulties faced by consumers getting their money back under website guarantees.
Video: MPs demand answers from Viagogo
It also raised concerns over speculative selling, when businesses advertise tickets they do not yet own.
The CMA said all sites must be clear if there are restrictions on using a resold ticket that could result in buyers being denied access to an event.
Its chief executive, Andrea Coscelli, said: “Secondary ticketing websites can offer an important service – by allowing people the chance to buy tickets at the last minute or giving them a chance to resell tickets they can no longer use.
“But our investigation has identified concerns that the law protecting consumers is being broken.
“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 regulatory inquiry is running alongside Government plans to crack down on excessive prices.
It was announced in the spring that touts who use bots to mine for concert tickets, before selling them for massive profits face, unlimited fines.
A number of artists including Ed Sheeran, Mumford & Sons, Radiohead and Amy Macdonald have encouraged fans to stop using secondary platforms Viagogo, Get Me In!, Stubhub and Seatwave in protest at prices.
Some acts have even cancelled thousands of tickets because they were not held by the original purchaser.