French prosecutors have opened an investigation into the allegations
Apple is set to face a grilling in the US Congress over allegations it slowed older phones to encourage consumers to purchase newer models.
The head of the Senate's commerce committee, Senator John Thune, has written to Apple's chief executive Tim Cook to demand an explanation for why Apple slowed down phones with flagging batteries and why it would not provide free batteries if older ones caused so much difficulty to devices.
Mr Thune's letter follows confirmation from French prosecutors that an investigation is being fronted by the finance ministry's fraud control department after Apple admitted slowing down old devices with low-capacity batteries.
"Apple's proposed solutions have prompted additional criticism from some customers, particularly its decision not to provide free replacement batteries," Thune said, according to a report in the Wall Street Journal.
Apple revealed it had "down-clocked" older models' central processing units (CPU) but said it did so to reduce the strain on dated batteries and stop the devices from unexpectedly shutting down.
At the time, the company asserted it would never "do anything to intentionally shorten the life of any Apple product, or degrade the user experience to drive customer upgrades."
Amid a wave of class action lawsuits over the "deceptive, immoral, and unethical" phone slowing, Apple issued an apology and vowed to be more transparent with customers over the capacity of iPhone batteries.
In a post on its website, it said: "We know that some of you feel Apple has let you down. We apologise."
The admission came after years of speculation from Apple customers that their older handsets were being slowed down in a bid to entice them into an upgrade.
Senator John Thune's letter asked Apple whether it would offer refunds to customers who had paid full-price battery replacements after the company admitted it was at fault over the slow downs and discounted replacements.
Apple did not immediately response to a request for comment.