A Roman coin discovered by a Brighton cleaner was struck at the time of Christ by a "Del Boy" forger who could not spell and did not know one emperor from another, it was claimed today.

Experts say the coin is a mystery because it is made from solid silver and probably cost the forger as much to make as he received in profit.

The British Museum has never seen anything like it and its rarity has pushed up its value from £100 for a genuine coin to at least £3,000.

The silver denarius is based on coins struck to commemorate the Battle of Actium between Octavian and the combined forces of Mark Antony and Cleopatra in 31BC.

But experts have revealed that the forger got most of his inscriptions wrong.

He crafted his denarius some years after the battle but had a poor memory of what the real coin looked like, it has been claimed.

On one side is a crocodile but it is facing the wrong way and on the other side is the head of Emperor Caesar when it should have been Augustus.

The forger made a further mistake by mis-spelling Egypt. He inscribed Aegipto instead of the common spelling of the time, Aegypto or Aegvpto.

Proceeds from any sale may now be used by its finder, 45-year-old Rob Clements, to study the Romans at the University of Brighton where he works as a cleaner.

Mr Clements discovered the coin lying two inches beneath the surface on a grass path in open ground on the outskirts of Brighton a few months after buying his first metal detector.

An examination using the university's new advanced electron microscope showed that the coin's content was solid silver.

Mr Clements registered the find with Laura Burnett, the finds liaison officer for Sussex, who sent pictures of the coin to the British Museum.

Sam Moorhead, national finds adviser for ancient coins at the British Museum, said the poor spelling "suggests the die cutter is not fully literate".

He said: "Interpreting the coin is difficult. Were it a (silver) plated piece, then it would have been explicable as an attempt to create a coin for profit by using a smaller amount of silver.

"However, why would someone create a fantasy piece like this in the ancient period from solid silver? As such, the coin is a mystery."

Mr Clements, who lives in Brighton, said: "I never thought I'd find anything so interesting and valuable and so soon after getting a detector.

"I would have been thrilled finding a genuine coin but this fake could mean a big difference to my life. I've always loved history but never bothered much at school.

"Now I'm seriously looking into the idea of selling the coin and putting it towards a degree here at the university.

"I hope to study more about the Romans. It's fascinating that there were forgers at the time, some, it seems, who were not very bright.

"I've always been a fan of Only Fools and Horses and it's amazing that I've connected with a Del Boy who lived 2,000 years ago."