An inquiry has been launched into claims that a woman went out on 999 calls after fooling ambulance staff into believing she was a trainee paramedic.
She reportedly walked into an ambulance station with a letter stating she needed to go out with crews as part of her training.
It is believed no-one checked her credentials and she worked at ambulance stations in Sussex for more than two weeks last month before it emerged she was an imposter.
One worker, who did not want to be identified, told the Brighton Argus: "It seems she was given a paramedic uniform to wear and went out on calls.
"I have also been told she was given the chance to help with treating one or two of the patients. It is rumoured that she went out to different types of emergency calls including ones to heart attack patients, stroke patients and car crashes, but the details are not known yet."
The worker added: "Luckily patients were not affected but it could have been a lot worse.
"It's clear security checks need to be improved. We need to make sure it doesn't happen again."
The woman was found out after another trainee paramedic alerted managers, saying the woman was not on the course she claimed she was on, said the Argus.
Officials at South East Coast Ambulance Service NHS Foundation Trust (Secamb) said there is no evidence that patients had been put at risk but they are treating the incident "extremely seriously".
Security codes at ambulance stations have been changed as a precaution and all staff have been reminded about security protocols.