Apple is facing questions from the US Senate over its practice of deliberately slowing down iPhones with older batteries, as the firm finds itself under pressure from both sides of the Atlantic.

Chair of the Senate commerce committee, Senator John Thune, has written to Apple’s chief executive, Tim Cook, questioning the company’s handling of iPhone slowdown and consumers.

“Apple’s proposed solutions have prompted additional criticism from some customers, particularly its decision not to provide free replacement batteries,” Thune said, reported the Wall Street Journal, requesting answers by 23 January.

Apple admitted in December to intentionally slowing down iPhones with older batteries, which spurred multiple class-action lawsuits and criticism from consumers. The firm apologised for its handling of the issue, saying that its intension was to “make iPhones last as long as possible”.

It said: “We have never – and would never – do anything to intentionally shorten the life of any Apple product, or degrade the user experience to drive customer upgrades.”

Thune questioned how Apple tracked and handled consumer complaints regarding slow iPhone performance, and whether Apple would offer rebates to customers who bought full-price battery replacements following the company’s discounting of the service from $79 to $29 (£25 in the UK).

Apple has reportedly faced shortages of batteries in its stores following the battery replacement programme. The slowing of devices is intended to avoid iPhones shutting down without warning when a worn battery can no longer provide enough current to run the processor at full speed, Apple said.

The pressure from the US follows action by the French government. The French fraud watchdog, which is part of the finance ministry, opened an investigation last week over alleged “deception and planned obsolescence”, following a complaint by a consumer rights group.

A 2015 French law makes intentionally shortening the lifespan of a product in order to encourage consumers to replace it illegal, with possible fines of up to 5% of annual turnover and jail terms of up to two years.

Apple did not respond to a request for comment.