A crackdown on online pricing has seen investigations opened into eight companies, with a further 100 facing warnings over their conduct.
The competition watchdog said it was formally examining practices at StubHub, viagogo, AA Driving School, BSM Driving School, Gold's Gym, Wayfair, Appliances Direct and Marks Electrical.
The Competition and Markets Authority (CMA) said the 100 other companies, which it did not identify, were getting letters outlining concerns about additional fees and sales tactics.
Money latest: Nationwide 'lost my £70k'
The action against StubHub and viagogo - part of the same company after a 2021 merger - was revealed as the government reportedly prepares to separately confirm a ban on the resale of tickets for live events above their face value.
It is part of a long-threatened crackdown on touts to shield consumers from rip-off prices.
The regulator's separate action falls under the new Digital Markets, Competition and Consumers Act which gives it additional powers to protect consumers.
The CMA said StubHub and viagogo were under review "regarding the mandatory additional charges applied when consumers buy tickets - and whether or not these fees are included upfront".
The AA Driving School and BSM Driving School were being investigated over their "presentation of mandatory fees on these sites", the CMA said, "specifically, whether these fees are included in the total price the consumer sees at the beginning of the purchase process."
Gold's Gym is under investigation over its presentation of a one-off joining fee for its annual membership, and whether the way it presents this fee breaks the law.
It explained that the examination of homeware retailers Wayfair, Appliances Direct, and Marks Electrical was related to whether their time-limited sales "ended when they said they would, or whether customers are being automatically opted in to purchasing additional services".
Commenting on the CMA's action an AA Driving School spokesperson said: "We are comfortable that the £3 booking fee for lessons is already transparent and in line with the CMA's rules and are more than happy to additionally notify customers earlier in the journey as well, which we have already done."
The other companies were yet to comment.
The CMA's first major act under the new digital market rules was to give itself special oversight over Apple and Google.
The CMA's so-called "strategic market status" rulings mean both companies will face specific obligations to limit their dominance in smartphone and tablet operating systems (iOS and Android respectively), app distribution and browsers.
Commenting on its latest inquiry, CMA chief executive Sarah Cardell said: "At a time when household budgets are under constant pressure and we're all hunting for the best deal possible, it's crucial that people are able to shop online with confidence, knowing that the price they see is the price they'll pay, and any sales are genuine.
"Whether you're spending your hard-earned cash on concert tickets or driving lessons, joining a gym or buying furniture and appliances for your home, you deserve a fair deal.
"It's our job to protect consumers from misleading prices and illegal pressure selling and today marks an important milestone as we take action across the economy to make sure businesses do the right thing by their customers."
"Since the launch of the new regime, we've been working hard to help businesses understand the law. But alongside supporting businesses to comply, we've always been clear that we will take swift action where we suspect potentially serious breaches of the law.
"This is just the start of our work. Any businesses who break consumer law should be in no doubt we will stamp out illegal conduct and protect the interests of consumers and fair-dealing businesses."
(c) Sky News 2025: Online pricing crackdown targets over 100 firms operating in the UK
Home secretary admits illegal immigration numbers still 'too high' under Labour - but says Farage can 'sod off'
Why women are being forced to go abroad to help heal from a lifetime of trauma
UK weather: Snow possible in parts of country today as temperatures plummet
Former pilot who tried to cut plane's engines after taking magic mushrooms won't serve more prison time
Sudan 'epicentre of suffering in the world', says UN humanitarian chief
Ticket resales to be capped at face value under government crackdown on rip-off prices
How a grassy Oxfordshire field became an illegal mountain of waste in months
New homes earmarked near train stations to get 'default yes' from planners