Ms Crighton-Smith, who was travelling on a train in south London, told the BBC's Victoria Derbyshire programme: "I had Airdrop switched on because I had been using it previously to send photos to another iPhone user - and a picture appeared on the screen of a man's penis, which I was quite shocked by.

"So, I declined the image, instinctively, and another image appeared, at which [point] I realised someone nearby must be sending them, and that concerned me. I felt violated, it was a very unpleasant thing to have forced upon my screen.`

"I was also worried about who else might have been a recipient, it might have been a child, someone more vulnerable than me.

"My name on Airdrop says Lorraine so they knew they were sending it to a woman. The images were of a sexual nature and it was distressing."

How to turn Airdop off

Airdrop is specific to iOS device and Apple Macs. It uses wi-fi and Bluetooth to talk over a short range to other devices, like other iPhones.

Its default setting is for "contacts only", which means only people you know can see you.

But if you want to share your information or your contacts with other people, you may make a change to the settings and change it to "everyone".

"This means that typically in a train carriage, or tube carriage, you can see other devices," commented Ken Munro, a cybersecurity consultant at Pentest Partners.

"That's what's happened in this particular case, someone has enabled everyone and then hasn't then set it back. As a result anyone within wi-fi or Bluetooth range can send something to you that's quite horrible."

He added that Apple could tackle the issue by making Airdrop return to its default setting if it had not been used for 10 minutes or so.

A spokesman for Apple declined to comment.