A woman who went for a lie down while suffering a chronic migraine woke to find that she appeared to be speaking in a French accent.

Kay Russell, 49, from Gloucestershire, said today she felt she had lost a part of herself after being diagnosed with foreign accent syndrome (FAS), an extremely rare condition that can be a side effect of severe brain injury.

Russell said friends she had known for years did not recognise her on the telephone and she had trouble convincing strangers that she was British. The former sales executive, who used to have an unmistakably English accent and has only been to France twice, said her confidence was shattered and she had to give up her job.

"A lot of people come up and say: 'What a lovely voice you have,'" said Russell, who has lived with the condition, of which there are only 60 recorded cases worldwide since 1941, since January. "You lose your identity and an awful lot about yourself. I feel like I come across as a different person.

"It's not just my voice I miss. I would love to have my old voice back obviously. But it goes way, way beyond that. It's the person I was – the person I want to be.

"I rang up a friend I had known since I was a teenager and the last time I had spoken to her I was speaking in my old voice. It took me a while to explain it was me."

Some people think that Russell is from eastern Europe or the former Soviet Union. But most fasten on French. Her condition has also changed her syntax. For example, she tends to say (and even write) "peoples" instead of people and misses out some basic words such as a, of and to.

FAS is an extremely rare condition which damages part of the brain controlling speech and the way words are formed. It can last for just a few days or be permanent.

Professor Nick Miller, an expert on FAS at Newcastle University, said: "A lot of people with foreign accent syndrome speak of a loss of their former accent or speak in terms of bereavement as though they have lost a bit of the their former selves. They say part of their personality has died almost or been lost to them."

Earlier this year Sarah Colwill, 35, a woman from Devon who used to speak with a West Country drawl, told how she began to speak in an accent that sounded as if she was from the far east. Paramedics who came to her help when she suffered from a migraine that apparently triggered FAS told her they thought she had a Chinese accent.