Ticket reseller Viagogo faces legal action after failing to make changes sought by the consumer watchdog.
The Competition and Markets Authority said rival resellers StubHub, GetMeIn and Seatwave had agreed to provide better information about the tickets sold on their sites.
They must warn if buyers risk being denied entry, which seats they will get and who is selling the tickets.
Viagogo has not given the same undertakings, the CMA said.
The site also failed to comply with a commitment given in 2015.
Michael Grenfell, the CMA's enforcement director, said it was determined to ensure that Viagogo complied with the law.
"We are prepared to use the full range of our powers to protect customers - including action through the courts," he said.
Sharon Hodgson, chair of the all party parliamentary group on ticket abuse, welcomed StubHub, GetMeIn and Seatwave's agreement to provide better information about the tickets sold on their sites.
A meeting of the group held on Wednesday heard "some truly shocking accounts of customers who have been ripped off by companies such as Viagogo, who are still refusing to comply", she said.
"The continued breaches of UK legislation by some secondary ticketing companies must be stopped."
After a year-long investigation into resellers that ended last November, the CMA said that some ticket resellers may be breaking the law by failing to tell consumers about restrictions on the tickets they were buying.
Alex Neill of Which? welcomed the CMA action on secondary ticket sites such as Viagogo, which had played "fast and loose with the rules".
"The agreements ... must now lead to much greater transparency, so consumers have a better chance of getting the best tickets for popular events at fair prices," she said.
The Fan Fair Alliance said forcing resellers to disclose the names of "traders" supplying the tickets was good news:
This is a good news @GETMEIN @Seatwave and @StubHubUK forced to change their business practices, and start disclosing names of "traders". But surely the sands of time are running out for @viagogo #toutsout https://t.co/trzdLfyz4y— FanFair Alliance (@FanFairAlliance) April 25, 2018
Reselling tickets for top-level football matches in England and Wales is illegal, with fans allowed to exchange them at face value through official club brokerages.
Many in the music industry have criticised resellers, with the manager of the Arctic Monkeys calling on the government to shut down Viagogo after tickets for the band's upcoming UK tour appeared on the site for as much as £2,200.
Ian McAndrew told BBC Radio 4's You & Yours programme the band had successfully stopped tickets appearing on Stubhub, GetMeIn and Seatwave.
"Viagogo should follow the example of other sellers or efforts should be made to remove them from the business," he said.
"The band shares the same feelings and views echoed by the fans, feelings of anger and frustration. There's a reputational issue here.
"It reflects badly on the group even though they're not responsible for that problem. Sometimes that frustration is expressed towards them and that's understandable."
Last month, the Advertising Standards Authority ruled that sites must make the total ticket price, the VAT-inclusive booking fee and the delivery fee clear from the start.
Resellers have been accused of misleading fans by claiming they are official sellers of tickets, overcharging buyers or selling tickets that are invalid if they are resold.
Viagogo is run by American Eric Baker. He founded StubHub with university classmate Jeff Fluhr and they sold the company to eBay in 2007 for $304m.
Mr Baker then moved to London to set up Viagogo, which is owned by a Delaware-based venture called Pugnacious Endeavors.