British consumer group Which? will launch legal action against Apple on Thursday, alleging the tech giant breached competition law by effectively locking millions of its customers into its iCloud service.

Which? said Apple encouraged its users to sign up to 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 had led to consumers being overcharged, by up to 13.36 pounds ($16.98) this year, through their monthly iCloud subscription.

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

"We reject any suggestion that our iCloud practices are anticompetitive and will vigorously defend against any legal claim otherwise," it said.

About 40 million Apple customers in Britain who have obtained iCloud services over the last nine years could be entitled to a payout if the claim is successful, Which? said.

Its Chief Executive Anabel Hoult said the group believed Apple customers were owed nearly 3 billion pounds as a result of the tech giant forcing its iCloud services on customers.

"Taking this legal action means we can help consumers to get the redress that they are owed, deter similar behaviour in the future and create a better, more competitive market," she said.

Which? said it would file the claim with the Competition Appeal Tribunal.