The FBI has warned that the convincing emails appear under an official looking header and inform US visa holders that they are among 50,000 winners selected for permanent residency as part of the Diversity Immigrant Visa Program, otherwise known as the Green Card Lottery.

"We always advise people that if it looks too good to be true, it probably is," said a spokesperson for the FBI.

"We have received complaints about this particular scam that claims to be from the US government. If someone receives these emails unsolicited, they should be treated with a healthy amount of scepticism."

Ian Jopson, an Australian citizen living and working in the US and a recipient of the scam email, said he quickly figured the scam out.

"The header said 'Congratulations, you've won!', I was overwhelmed because I've been in the [Green card] lottery for years," said Mr Jopson. However, when the email requested almost $900 (£566) in 'processing fees' to be sent to an 'embassy representative' in London via Western Union, Mr Jopson knew the email wasn't legitimate.

"It was a red flag, it's general knowledge that Western Union is used for these things."

Mr Jopson said the email requested a confirmation message be sent after payment had been made which made it clear the email was a scam. He said the address 'lottery[at]usa-green-card-dv-lottery.com' was clearly a scam as any official email address would end in '.gov'.

Jason Stern, a New York attorney who specialises in online cases, said the scam wasn't particularly sophisticated but became more dangerous because it specifically targeted immigrants, traditionally considered a vulnerable demographic due to the desperation many of them have to remain in the US.

"The Green Card Lottery scam is really no more sophisticated than your garden-variety Nigerian bank email phishing scam, except that it specifically targets who are particularly vulnerable to a message bearing the official looking seal of the US Department of State," said Mr Stern.

A spokesperson for the State Department said the department had since ceased sending out emails regarding the Green Card lottery altogether due to the email to prevent people falling victim to the scam.

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The American Visa Bureau is an independent migration consultancy specialising in helping people from Visa Waiver Program (VWP) countries make their ESTA application.