Police said the gangs behind the fraud were frequently involved in other forms of organised crime and the funds used to finance drug trafficking. They are now using e-mail and faxes to target potential victims.
The sting takes the form of an invitation from senior government officials or members of the Nigerian Royal family to participate in what purports to be a foolproof money-making scheme.
Typically those who respond are not asked to provide money up front but are invited to a series of business meetings which add to the plausibility of the scheme. Only later when they are told that a Nigerian company has to be set up to channel the funds are they asked for money, typically $10,000. They then find they have to stump up more for lawyers, accountants and other advisers.
The Independent has obtained a copy of an invitation which was faxed to the address of a former doctor a few days ago. It is signed Dr Suleimon Idris, who claims to be a member of the Contract Award Committee of the Nigerian Petroleum Corporation.
The letter claims Dr Idris is in possession of $22.5m in a dedicated account with the Central Bank of Nigeria Debt Reconciliation Committee, which he has been authorised to transfer into a foreign account. The recipient is invited to allow his own or his company's account to be used for that purpose, in return for being able to keep 25 per cent of that sum plus a further 5 per cent to cover expenses.
All the recipient has to do is provide their account details. The letter concludes: "Please treat this transaction as STRICTLY CONFIDENTIAL as we are civil servants who would not want any exposure."
Dave Cooper, an officer with the National Criminal Intelligence Service, said the victims were typically small businessmen. "People do fall for it. A high proportion tend to be small businessmen who have dreams of making easy money."
Last year around 600,000 of these letters were intercepted by police forces worldwide, 215,000 in the UK alone.
Mr Cooper believes most victims do not come forward because of the "embarrassment factor". "This is just the tip of the iceberg," he said. "This is huge. We estimated that about 10 per cent of those who receive these letters answer them while 1 to 2 per cent actually hand over money. "We have documented cases that have lost more than pounds 1m. It is not uncommon to encounter victims who have lost pounds 50,000-pounds 100,000."
The names used are often fictitious, although occasionally real names are used without the knowledge of the individuals concerned.
Since the scam - known as advanced fee fraud or 419 fraud after the article in the Nigerian criminal code which covers these activities - surfaced several years ago, a number of arrests have been made in the UK, continental Europe and the United States, but the signs are that the fraud is still growing.
As more people get wise to it, the fraudsters are, say police, increasingly resorting to foreign intermediaries to disguise the Nigerian connection.