People are being encouraged to check their change for “extra-special” 50p coins that sold for £2,500.

Two commemorative 2012 Olympic 50p coins - marking the London games with images of swimmers on the tail side - sold for thousands on eBay this week.

Attracting attention from keen collectors, the pair, one with lines covering the swimmers face and one without, saw a late rush which added more than £1,000 to their value, eventually selling for £2,500 after 12 bids. The coins were designed in 2011, with the Royal Mint explaining: “The design was created by Jonathan Olliffe, and it depicts a swimmer in action with fluid lines struck into the metal representing the waves.

“In the rare first design, these lines covered the swimmer’s face. Soon after its release, the coin was tweaked to make the face more visible, but not before the first design made it out into the world. One of the most exciting aspects of this special 50p is that no one knows exactly how many entered circulation before the changes were made, only that numbers are extremely limited.

“This is often the case with error coins, and they immediately become a collectable piece. Few have ever been sold and many take pride of place in private collections, but there’s always a chance this valuable coin could find its way to you.”

The 50 pence piece has become the most valued and collected coin in the UK, with many collectable designs appearing on its heptagonal canvas. Its 27.5mm diameter makes it the largest of any British coin and allows space for decorative pictures.

It has often been used to celebrate big events over the past 50 years of British history. The rarest coins tend to be of the greatest value, with the mintage (number of coins with each design made) being the fundamental attraction for collectors.

Along with the design, other aspects of the coin which increase value are the condition of the coin and whether it has an error in its design. The way in which it is sold can also determine the coin’s value - while some coin collectors will bid vast amounts of money on ebay or at auction, others opt for more robust valuations by selling via a coin dealer.