Gardener Bill Battaglia could hardly believe his eyes when he received a letter offering him a 30 per cent cut of a $21.5 million deal.

The letter, which came from a man claiming to be Prince Kalu Adamu of Nigeria, offered him the guaranteed windfall in exchange for his account details.

Mr Battaglia has run Stratton Garden Contractors single-handedly for 20 years, and has never had any contact with Nigeria.

So he was surprised when an envelope with a Nigerian postmark on it appeared on his doorstep with his address handwritten on the front.

The letter, headed "Strictly confidential", asked for Mr Battaglia's help in transferring the huge sum into a "reliable" foreign account.

The Prince, who claimed to be the director of project implementation at Nigeria's Ministry of Petroleum Resources, suggested the money could be split, with 60 per cent going to his ministry, 10 per cent on tax and the remaining 30 per cent about £4.5 million going to Mr Battaglia.

But the Nigerian High Commission in London has confirmed the letter is a persistent scam which has netted similar fraudsters more than £15 billion over the last 10 years.

Mr Battaglia, 44, from Kingsdown Road, said his first thought was that it was a practical joke by a friend, but admitted he was tempted by the offer.

"I'm just a humble tradesman," he said. "I have no idea how they got my address or why they chose me.

"I did think about responding when I worked out the figures who wouldn't be tempted by £4.5 million? But the chances are it is some corrupt operation and I'd have ended up losing out."

The letter asked Mr Battaglia to contact the Prince on his direct telephone number to confirm his willingness to assist him.

Speaking to the Evening Advertiser, the Prince defended the deal. "It is true that there are corrupt people in my country and not everybody in our society is 100 per cent good," he said.

"But we are not party to that sort of thing we have worked for the Nigerian Government for 25 years and have a reputation to protect."

But Greyne Anosike, the press attache at the Nigerian High Commission in London, said it was definitely a fraud stunt.

"The Nigerian Government has spent a lot of money advertising in the international press to advise businesses to ignore any unsolicited business deals from Nigeria. This is certainly a scam.

"These people basically get access to people's accounts and then syphon off every penny and that is the last you hear from them."

Robert Taylour, of Swindon Trading Standards, warned any other recipients of the letter to bin it. He said: "I'm not aware of anyone locally who has been caught out by this, but I would advise people to be very cautious because this sort of thing has been going on for years."