Transatlantic passengers on Aer Lingus flights who ignore repeated requests to turn off their phones or switch them to flight mode risk being hit with sky-high roaming charges even if they never look at their devices while on board the plane.

One passenger who contacted The Irish Times said he had inadvertently left his phone on in an overhead compartment while flying to the US and was horrified to get a bill for almost $300 from AT&T, his provider, weeks later.

AT&T said the charges were racked up by “antennas installed on the plane that operate outside an unlimited international roaming plan”. The antennas can “automatically connect with phones that are not in flight mode and run up charges – even when the phones are not in use”.

Aer Lingus confirmed that passengers who failed to switch their phones to flight mode could be hit with unexpectedly high bills but stressed it was not making any money as a result. It said it could not be held accountable if passengers ignored clear instructions.

“For safety reasons, before every flight, Aer Lingus cabin crew advise guests to switch their phones to airplane mode,” a spokeswoman said.

She added that passengers could use in-flight wifi on transatlantic flights if they registered and paid a fee but if a phone was simply left on it “may connect to the in-flight roaming network and the guest will be billed by their home operator for any usage”.