Ticket resale site Viagogo is facing court action by the competition watchdog for allegedly breaking consumer protection laws.
The Competition and Markets Authority (CMA) said Viagogo had failed to make changes to bring it in line with the law, despite warnings.
Following an investigation into the secondary ticketing industry, the CMA began enforcement action against four websites last November.
Three – StubHub, GETMEIN and Seatwave – agreed to make changes.
The CMA says Viagogo is breaking the law because shoppers are not being told whether there is a risk they will be turned away at the door because of restrictions placed on some resold tickets.
Its other concerns include consumers not being informed which seat they will get; misleading information about the availability of tickets; difficulty in getting money back and being offered tickets that the seller does not own.
CMA boss Andrea Coscelli said: "People who buy tickets on websites like Viagogo must be given all the information they are entitled to.
"It's imperative they know key facts, including what seat they will get and whether there is a risk they might not actually get into the event, before parting with their hard-earned money.
"This applies to Viagogo as much as it does to any other secondary ticketing website.
"Unfortunately, while other businesses have agreed to overhaul their sites to ensure they respect the law, Viagogo has not. We will now be pursuing action through the courts to ensure that they comply with the law."
The regulator said it was seeking an interim enforcement order from the court to "put a stop to some practices" until the full trial can be heard.
Viagogo was not immediately available for comment.
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In March, the Advertising Standards Authority (ASA) banned four websites from giving "misleading" price information. It also banned Viagogo from claiming to be the "official site" and offering a "100% guarantee".
And in May, Viagogo was referred to Trading Standards by the ASA for failing to make changes to pricing information on its website.
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