Secondary ticketing website Viagogo is "breaking UK advertising rules" by failing to make additional fees clear, a watchdog said.
The Geneva-based firm was one of four who were subject to Advertising Standards Authority (ASA) action in March after a clampdown on "misleading" pricing information.
StubHub, Seatwave and GetMeIn were the other three companies subject to the action against so-called "drip pricing" - where VAT, booking and delivery fees were added at the end of the booking process.
The ASA received 23 complaints about Viagogo, who were contacted by its sister organisation the Committee of Advertising Practice (CAP) for assurances the company would amend its website.
A statement on the ASA website said: "Viagogo provided that assurance on 23 March 2018, agreeing to make compulsory fees sufficiently clear by 26 May 2018.
"Nevertheless (as of Tuesday) the website www.viagogo.co.uk continues to include pricing information that is in breach of the CAP Code and the ASA's ruling."
Campaign group Fanfair Alliance, who campaigned against secondary ticketing online, and the music promoters Festival Republic and Kilimanjaro Live were among those who complained to the ASA about issues including additional ticket fees.
According to the ASA, Viagogo said "each ticket home page contained the text 'prices exclude booking and delivery fees (applicable by transaction)', and therefore at the first stage of the customer journey, consumers were informed that the prices displayed did not include additional booking and delivery fees".