Police are warning traders to be on the lookout for the coins, which are such good copies that they can only be detected because they are magnetic.

A number of the coins have turned up in Londonderry after one was found stuck to some copper money in a parking ticket machine.

According to Constable Andy Lindsay, a crime prevention officer at Strand Road Police Station in Derry, the counterfeits are fairly sophisticated.

"The dud coins are very good and are almost impossible to detect just by looking at them," he said.

"The counterfeiters have made an impression of the 'tails' side of the coin and attached that to a smaller copper coin, probably an Isle of Man or Channel Island penny.

"That penny is then enclosed in a 'jacket' made up of the 'heads' side and milled edge of the coin.

"While visually they are good copies, the counterfeit coins can be detected.

"If they are shaken, the penny will rattle inside the 'jacket'.

"And, a definitive test, is that the duds are magnetised, so that copper coins will stick to them."