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Soaring demand pushed the price of a rare 20p piece up to £20,000 on eBay this week.

Several are hoping to get more than £10,000 for their 20p. One seller appeared to have fetched £7,100 at 2pm yesterday - 35,500 times its face value - although it is not clear whether the coin has been paid for.

On Wednesday one potential buyer started bidding off at £600, soon followed by a bid of £800. Then it shot up to £5,000 and kept rising until the price reached £7,100. A spokesman for eBay said: 'The winning bid was definitely £7,100, but I can't confirm yet whether the actual transaction has been completed yet.'d without a date mark, could be worth £50 each.

But collectors have been scrambling to buy the currency after it emerged that the Royal Mint made an error when producing tens of thousands of 20p pieces earlier this year. The coins were left dateless when the 20p piece was redesigned.

The Mint does not know how many undated coins were released into circulation, but estimates range between 50,000 and 200,000.

This is the first undated British coin to enter circulation in more than 300 years - the last occasion was 1672, when Charles II was on the throne.

The error has caused a great deal of excitement among coin collectors because the Royal Mint, in Llantrisant, South Wales, is normally so careful about the manufacture and release of coins into circulation.

The problem occurred after all the coins from the 1p through to the £1 were redesigned last year. The designs for the 5p and 10p had been unchanged for four decades.

The Royal Mint's usually impeccable quality control somehow slipped up and the so-called 'mule' escaped into circulation. A 'mule' is a coin that has mismatched sides - a reference to the mule being a mismatch of a horse and donkey.