Shares in Trainline fell after the U.K. antitrust regulator launched an investigation into pricing practices at the U.K. rail-and-bus ticket website.

In morning European trading, Trainline shares were down 14% at 2.08 pounds. Year to date, shares have fallen around 5%.

The Competition and Markets Authority said Wednesday that it has launched three new investigations into Trainline, Virgin Atlantic and RED Driving School, citing concerns that the total price might not be clearly disclosed upfront to customers when buying train and coach tickets, holidays or driving lessons. These investigations are part of a broader CMA clampdown on potential misleading pricing practices.

Trainline is being investigated over whether all mandatory fees are included in the upfront prices shown to consumers booking train and coach tickets in advance on the Trainline app and website, the CMA said.

The Virgin Atlantic investigation will focus on whether mandatory resort fees and local taxes are included in the upfront package-holiday prices. RED Driving School is also under investigation over how mandatory booking fees and 'digital' fees are displayed when customers book driving lessons, it added.

"All firms under investigation were sent 'advisory letters' as part of the CMA's first consumer protection drive using its new enforcement powers," the regulator said.

The CMA is still in the early stages of its investigations and hasn't reached any conclusions as to whether these companies have broken the law, the regulator said. In case of infringement, the CMA could order businesses to pay compensation to affected customers and impose fines of up to 10% of global turnover.

Trainline said it has proactively engaged with the U.K. regulator over several months, while taking steps to enhance the presentation of certain fees.

"We will continue to work constructively with the CMA to ensure our customer experience remains both transparent and compliant," Trainline said.

"We take our responsibility to customers incredibly seriously and we always want to ensure they can make an informed decision when booking a holiday with us," Virgin Atlantic Holidays spokesperson said. "Mandatory fees are indicated at multiple stages throughout the booking journey."

RED Driving School didn't immediately reply to a request for comment.