UPDATE: the watchdog is preparing a legal case against ticket reseller Viagogo.
The Competition and Markets Authority (CMA) says that, despite warnings, Viagogo is still not following a court order requiring it to comply with consumer law.
The CMA gave Geneva-based Viagogo until 17 January 2019 to overhaul its practices to make buying secondary tickets clearer.
But it says checks of the site have shown there are still 'issues of concerns'.
In a statement, the CMA said: 'For a company not to comply with a court order is clearly very serious.
'We are therefore now preparing to take legal action to ask a court to find Viagogo in contempt.'
Which? consumer rights expert Adam French said: 'We have repeatedly exposed Viagogo as a rogue operator flouting consumer law - so it is right for the regulator to take action and send a clear message that there can be no hiding place for unscrupulous companies if they are found to have ripped off British consumers.
'Until Viagogo can show that it is prepared to implement these important protections, we would advise people buying tickets to steer well clear of the company.'
27 November 2018: Secondary ticketing site Viagogo has been ordered to change its practicesafter a landmark court ruling.
We have long been calling for a clampdown on secondary ticketing sites' practices and we welcome this ruling.
Under the enforcement order issued by the high court, by mid-January Viagogo will have to tell ticket buyers:
Viagogo must also make it easier for people to get their money back when things go wrong and prevent the sale of tickets which the seller doesn't own and can't supply.
After investigating the secondary ticketing sector, the CMA warned the four major secondary ticketing websites to overhaul how they did business and respect the law.
StubHub, Getmein! and Seatwave all offered formal commitments but Viagogo continued its business as usual.
In August, the CMA launched legal proceedings in the high court and sought an interim enforcement order to stop some of Viagogo's practices.
The mid-January deadline to comply with the court order is the same timeline given to the other secondary ticketing sites that have already agreed to change their practices.
The court order means there is no need for a trial.
It's the first time Viagogo has appeared in the UK courts over its practices.
If the site fails to comply with the order, the courts could fine the company and fine or imprison certain individuals.
Which? managing director of home products and services, Alex Neill, said: 'It's right that Viagogo is being forced to make these changes after we repeatedly exposed the company as a rogue operator for flouting consumer law.
'It is shocking, however, that the company had to be dragged through the courts to ensure music and sports fans are given the information they are entitled to when buying tickets.
'Viagogo should immediately take steps to overhaul its practices so its customers are no longer misled.'
The chief executive of the CMA, Andrea Coscelli, said: 'This court order is a victory for anyone who decides to buy a ticket through Viagogo.
'We have been clear throughout our investigation that people who use these resale websites must know key facts before parting with their hard-earned money, including what seat they will get and whether there is a risk they might not actually get into the event at all.'
If you buy an event ticket from a secondary ticketing site, you must be told: