The competition watchdog says it will "not hesitate" to take action against Viagogo if the ticket resale site fails to comply with a court order to identify touts.
From Friday, Viagogo must publish the names and addresses of touts, those who sell more than 100 tickets a year via the site, to consumers.
Viagogo said it has already met the deadline and is "now compliant".
Claire Turnham, awarded an MBE for obtaining close to £1 million of refunds from the site through her Victim of Viagogo group, said:
But details already emerging about some sellers using the site caused her concern, she said.
In November, the CMA told the Switzerland-based company it must overhaul the way it does business to protect consumers under a High Court order.
Under the order, Viagogo must also warn buyers if the tickets are subject to a ban on resale that might lead to them being turned away at the door, and publish information such as the face value of tickets and the seat number.
It must also not give misleading information about the availability and popularity of tickets, which the CMA said had the potential to lead to customers rushing into making a buying decision or making the wrong choice.
The order will also make it easier for consumers to get their money back under Viagogo's guarantee when things go wrong and ensure that the site does not repeat historic failures to make its customers aware of the face value of tickets, the watchdog said.
Viagogo is subject to the same deadline set for other resale sites that have already agreed to change their practices.
Competition and Markets Authority chief executive Andrea Coscelli said:
The CMA began enforcement action against four secondary ticketing websites last November, with three of them - Stub Hub, GETMEIN! and Seatwave - committing to change their business practices.