LONDON (Reuters) – British consumer group Which? will launch legal action against Apple on Thursday, alleging the tech giant broke competition law by effectively locking millions of customers out of its iCloud service.

Which? said Apple encouraged its users to sign up for iCloud to store photos, videos and other data, while making it difficult to use alternative providers.

Customers then had to pay for the service once their data exceeded the free 5GB limit, it said.

Which? said the resulting lack of competition has led to consumers being overcharged by up to £13.36 ($16.98) this year on their monthly iCloud subscription.

Apple said in a statement that its customers were not forced to use iCloud, and many used third-party alternatives.

“We reject any suggestion that our iCloud practices are anti-competitive and will vigorously defend against any legal claim,” it said.

Around 40 million UK Apple customers who have obtained iCloud services over the past nine years could be entitled to a payment if the claim is successful, which? said.

Chief executive Anabel Hoult said the group believed Apple customers were owed almost £3 billion as a result of the tech giant forcing its iCloud services on customers.

“Taking this legal action means we can help consumers get the compensation they are owed, deter similar behavior in the future and create a better, more competitive market,” she said.

Which? said he would submit the application to the Competition Appeal Tribunal.

($1 = £0.7870)

(Reporting by Paul Sandle; Editing by Michael Holden)