Britain's competition regulator is investigating whether Virgin Atlantic, Trainline (TRNT.L) and RED Driving School were transparent in disclosing total pricing when customers made bookings, it said on Wednesday.
Shares in Trainline, the only listed company of the three firms, were down nearly 17% to 202.4 pence at 1108 GMT, underperforming the wider market.
The UK Competition and Markets Authority said it launched the investigation after it received letters raising concerns that customers had not been shown the total price upfront when buying train and coach tickets, holidays or driving lessons.
The regulator said its investigations were at an early stage and had yet to reach any conclusion on whether the firms had broken the law.
Rail and coach ticketing operator Trainline said it has proactively engaged with the CMA over several months and is taking steps on how it presents certain fees, adding it will continue to work to ensure transparency.
British carrier Virgin Atlantic said it was reviewing the concerns and will cooperate with the regulator.
RED said it was disappointed to be named in the investigation, but added it would continue to engage with the CMA through the probe.
The watchdog said that if it finds that the businesses had broken the law, it could order them to pay compensation to affected customers, as well as fine them up to 10% of their global turnover.
"Clear pricing helps people compare offers confidently and choose the option that works best for them. Unexpected mandatory charges make this much harder," said Emma Cochrane, executive director for consumer protection at the CMA.
"At a time when many households are watching every pound they spend, it is important that people are not surprised by extra fees."
