Energy company Vattenfall faces a mass claim from customers after a court ruled it had unfairly raised its tariffs when gas prices increased sharply following the Russian invasion of Ukraine.

The company put up its variable rates in April 2022 in response to rapidly inflating prices on the international gas markets, which quadrupled between January and August.

But the appeal court in Amsterdam said Vattenfall’s decision to hike prices in April was an “unfair trading practice” because the conditions of its variable contract said its rates would change twice a year, in January and July.

Vattenfall claimed its customers were aware when they signed their contracts that rates could change at any time. They were also free to switch to a different supplier without penalty at 30 days’ notice.

But the court rejected the argument, potentially paving the way for a wave of claims by other customers.

Stichting Eerlijke Handelspraktijken, which specialises in mass claims in consumer cases, said all 4.5 million people who were on variable contracts in 2022 could potentially be entitled to compensation.

However, it warned that Vattenfall is highly likely to appeal to the Surpreme Court. The company said in a response that it “fundamentally disagreed” with the appeal court’s ruling, insisting that its terms and conditions “clearly stated” that prices could vary if prices rose on the wholesale markets for gas and electricity.