An American woman has revealed how she was swindled out of $400,000 (£269,000) by Nigerian internet fraudsters, in what is believed to be one of the biggest cases of its kind ever recorded.

Janella Spears, a registered nurse from Sweet Home, Oregon, said she started sending money to the scammers in 2005 after she received an email promising her several million dollars from a long-lost relative. In what is commonly known as a 419 scam - named after a section of the Nigerian criminal code - the fraudsters randomly contacted Spears over the internet, claiming they would offer her a substantial cut of $20.5m fortune in return for the cash injection which would help move it out of the country.

"I kept thinking it's only a couple hundred dollars - I can get it back," she told local news. Over a period of two years, the fraudsters strung her along and encouraged her to send more payments of up to $14,000 at a time. In the end she became obsessed and sent the fraudsters more than $400,000, which she raised by remortgaging her home and spending her husband's retirement savings.

Despite advice from bank officials, police and even the FBI that the scheme was a ruse, Spears said she continued to send cash in the hope of a large pay-off. Even fake emails claiming to be from the President of Nigeria and US president George Bush could not dissuade her.

"I said how come you're using this non-government address? 'Oh, because our computer has a worm'," she said. The 419 fraud is one of the most common internet deceptions, and like most similar schemes is reliant on sending millions of spam messages in the hope that they land in the inbox of a gullible victim.

Although finding victims requires luck rather than judgement, the incredibly low cost of sending email means that such fraud can be highly profitable. A study by researchers at University of California showed that it took an average of 12.5m spam emails for each response - but that large operations sending billions of messages could make as much as £2m a year.

Although Spears is an exception she is far from being alone. Last year British police helped crack a similar fraud ring, which was holding fake cheques worth more than £1bn, which had links to groups in the Netherlands and Nigeria.

Although it is difficult to calculate the value of such swindles because many victims are too embarrassed to report the crime, the government has estimated that such frauds could cost £3.5bn a year in Britain alone.

Spears claimed she was sharing her story now in order to prevent others from falling victim to similar scams. "You're sitting there going 'how can I fall for something like that'," she said.