We urge people to be on their guard.

Duncan McKelvie, NCIS

The correspondence appears to be from an "official" foreign government or agency which offers to transfer millions of pounds into the person's personal bank account.

One victim of the scam told the BBC she had been promised �2m for the use of her bank account if she put up �10,000 in expenses.

"Five thousand dollars was used to open an account," she said.

"The remainder of the money was used on the pretence that a certificate was needed to transfer the money out of the country. It was also to pay for an armoured vehicle to transfer the funds from a security company to a bank."

Once the money is paid, the promised cash fails to materialise from abroad.

Change of tactics

The scam, which started in Nigeria, is known as the 419 Advanced Fee Fraud after the section of the Nigerian penal code which covers the crime.

But the NCIS said recent successes in combating the fraud, which had spread throughout West and sub-Saharan Africa, had meant that criminals were changing their tactics.

Changes included posting the fake letters from different source countries, such as Gabon, South Africa and Botswana, and sending some letters in bulk to Britain, where they were then posted.

Duncan McKelvie, of the NCIS West African Organised Crime Section, said many people would be taken in by the fraud.

He said millions of pounds were invested in this type of fraud.

Losses prevented by NCIS

Scotland: �63m Northumbria: �6.8m West Midlands: �27.2m South Yorkshire: �15.5m Greater Manchester: �1.5m

"The criminals involved in the sending of these letters are also involved in many other crimes; drugs, cars, people, the transportation of human beings for instance," he said.

"The money that people send unknowingly for this fraud is often used to fund that criminality."

Victims were often too ashamed to come forward and some were suicidal when they realised they had been duped into parting with thousands of pounds.

Police say the number of letters seized over the last three years represented estimated potential losses of �24m.

'Gullible people'

The Nigerian Government has set up a unit in London to help fight the fraudsters.

But Nigerian High Commissioner Bola Ajibola said the people falling for the scam were as culpable as the fraudsters.

"It is unfortunate that we have some of these gullible people who can easily get targeted with the idea of a lot of money somewhere," he said.

"I think it is not only stupid for people to do that but they are what we call "criminous participants", they are also criminals themselves."

Anyone receiving such a letter or e-mail is advised not to reply and forward the correspondence to the NCIS, Post Box 8000, London SE11 5EN, or by email to [email protected]