Amsterdam-based online travel agency Booking faces a mass claim law suit launched by Dutch consumer group Consumentenbond over its ways of influencing consumer choices.

Booking earned around €1 billion as a result of so-called “dark patterns”, using fake discounts, incomplete pricing and made up scarcity, the Consumentenbond says. “That is illegal according to the Dutch and European rules,” director Sandra Molenaar said.

Bert Heikens, chairman of funding foundation CCC, said that consumers have been paying too much for their hotel rooms for years.

“Our investigation shows that Booking has been contravening competition and consumer rules since 2013. Via illegal deals Booking is preventing hotels from offering lower prices or better conditions via their own websites,” he said.

The Consumentenbond and CCC are calling on duped customers to file their claim, including those who used the hotel websites and other accommodation sites such as Trivago because Booking kept the prices in the whole sector artificially high.

Booking will be offered the opportunity to honour the claims in an out-of-court settlement. If not, the case will go ahead, the Consumentenbond said.

Booking told Dutch News in a statement that the company has “always fought to provide the best possible prices and transparent booking experience to travelers and strongly contest any allegations to the contrary.”

“We are reviewing the contents of this claim and will respond accordingly,” the statement said. “We have removed price parity clauses across Europe, but believe that they did help ensure consumers could access great prices everywhere.”

Last year, Spain’s competition watchdog fined the company €413 million for abusing its dominant market position in the country for the last five years.