The superdollar is the name given to an almost perfect forgery of an American banknote. Millions of dollars of the fake cash have been passed into circulation since its existence was first noticed over a decade ago.

The money, officially known as Note Family - C14342, is thought to originate from communist North Korea.

Experts believe that the money is being produced and flooded into the system, mostly by North Korean diplomats as they travel abroad.

It is also circulated by criminals - with the Russian mafia and even Republican organisations in Northern Ireland involved in the distribution process.

Sophisticated

One school of thought is that it is part of a plan to try to destabilise the American economy by putting millions and millions of dollars into the system.

However, North Korea dismisses the allegations and says that the claims being made against it are just Western propaganda.

What is known, is that in the late 1980s, US Intelligence discovered that the North Korean government had acquired a highly sophisticated printing press, known as the intaglio.

This press is similar to the one used to print money in the US and would give North Korea the ability to produce sophisticated banknotes if they wanted to.

Further evidence comes from defectors, whose stories all seem to be consistent with US claims.

Although it is impossible to corroborate their stories, they appear to be consistent.

Real expertise

One defector who spoke to Panorama on the condition of anonymity said he had spent his life making counterfeit US dollars, adding that they were such good quality that they fooled experts.

He said: "The counterfeiting was all done at government level. We had a special plant for doing it.

"When I defected I brought some of these counterfeit notes to South Korea, and I showed them to the experts in the South Korean intelligence agency. They said - these are not fake notes. They're real."

Another defector said: "We bought the best of everything - the best equipment and the best ink. But we also had the very best people, people who had real expertise and knowledge in the field.

"When government officials or diplomats travelled to south-east Asia they distributed the counterfeit notes mixed in with the real one's, at a ratio of about 50-50."