Ryan Stanley (left) tells Sam Smith the cards are sold as novelties

The problem was discovered by the BBC South West Inside Out team while filming teenagers drinking wine, beer and spirits in an Exeter park.

The investigation has resulted in one supermarket chain retraining its staff in how to spot them.

One company which makes cards said that they were only made as "novelty items".

No questions asked

During Inside Out's investigation several children said they had fake ID cards. One 14-year-old girl showed reporters a fake card, claiming she used it "every day" to buy booze and cigarettes.

To show how easy it was to obtain the cards, presenter Samantha Smith got her 12-year-old son to apply for one online, giving a fake date of birth to "prove" he was 18.

He then sent off a passport photo and �10 cash. The card arrived two days later with no questions asked.

The team also tested eight shops to see if they were fooled by the cards, using a young-looking 18-year-old actress.

Three out of eight stores challenged her to prove her age, but they all accepted the fake card as genuine.

One company making such cards is Sumo Monkey Ltd in Macclesfield.

Its website boasts that one of its fake cards is more "successful" than the proof of age card designed by the Portman Group to help retailers check ages of customers buying alcohol.

One teenager said she used a card "every day" to buy booze

Website manager Ryan Stanley described the cards as "novelty" cards and people purchased them for "various reasons".

He said that illegal use of fake IDs "can happen", but that the firm was not going to stop selling them.

Presenter Sam Smith said: "It's illegal to use a fake ID card fraudulently, ie: if you are trying to pretend you are older than you really are. But incredibly, it is NOT illegal to make and sell them.

One of the stores which was fooled by the fake ID was a Co-Op supermarket in Plymouth.

Plymouth and South West Co-operative Society Corporate Relations Officer Paula Meagor said: "The investigation was a real eye-opener.

"These cards are worryingly convincing and we are now retraining our staff in how to spot them."

A full investigation report can be seen on Inside Out, BBC One South West at 1930 BST on Monday 6 September.