old pennyJeff Moore/Jeff Moore/Empics Entertainment

There are thousands of old pennies kept by nostalgic collectors in the UK. Most are worth £1 at the very most - even if they are in brilliant condition. The damaged ones are worth their weight in scrap. However, one is about to go on sale, and is expected to fetch at least £2,500.

So why is it worth so much, and how does it compare to the most expensive coins?

The penny

The Penny is incredibly rare, because it was made in 1919 out of the wrong kind of metal. It's made from a blend of copper and nickel instead of bronze, and because it's the only one of this type that the experts know of, it has real rarity value.

According to the Daily Mail, it was made by the Kings Norton Metal Company, which at the time was making UK pennies, but also British West Africa pennies - which were made from the copper and nickel mix.

The manufacturers accidentally used a blank from the British West Africa pennies to make a British penny. It made two pennies, but while one went into the fire, the other was kept by the man who spotted it. He passed it to Francis Edward Brown, who wrote a note explaining the history of the penny. The note and the penny will be sold by Woolley and Wallis in October.

How does it compare?

It seems like a rare oddity, but it's small potatoes in the world of pricey coins.

The world's most valuable coin is the Flowing Hair dollar - the first ever issued by the US federal government. It was made in 1794, and the coin believed to have been the first one struck was sold at Auction in New York for $10 million in January.

Second place goes to the 1933 Double Eagle, with Liberty on the front and a flying eagle on the back. Just after half a million of the coins were struck from gold, a law was passed banning the payout of gold, so they were never circulated. Most were destroyed, but one remains, and is worth $7.5 million.

In third place is the 1787 Brasher Dubloon, which sold for $7.4 million. These were privately minted gold coins of which this was a rare and expertly-made version. It was bought by a Wall Street investment firm.

Even closer to home, coins can be much more valuable than our rare penny. The Double Leopard, made in 1344, is worth £460,000, as only three are left in existence. It's the most valuable UK coin.

And who knows, there could be a treasure like this lurking in your loft. In January the Woolley & Wallis auction house sold a rare American Half-Penny from 1796.

It was found in a homemade 'cabinet' of decorated matchboxes, glued together by English schoolboy Mark Hillary over 50 years ago and remained forgotten at the back of a cupboard after he tragically died at an early age of 20 in 1963 in a climbing accident in Greece. The coin eventually sold for £225,700 and Daniel Fearon, Woolley & Wallis coin specialist who identified the coin amongst the collection commented: "It was a wonderful result and a most satisfying ending to an amazing find."