When was the last time you checked your loose change? You could have an "incredibly rare" 50 pence piece that's attracting bids of up to £310 on eBay.

The Royal Mint is encouraging people to check their purses, wallets, jam jars and sofas for one of 210,000 Kew Gardens coins (see the picture on the right) released into circulation in 2009 to mark the 250th anniversary of the Royal Botanic Gardens.

It bears the years 1759 and 2009 and says "Kew" at the base of a pagoda, which is encircled by a twisting vine plant.

The rarity of the coin is attracting collectors, with bids of up to £310 for one in its original packaging and bids of around £60 for loose coins, being made on auction site eBay.

About one in 300 people are thought to have at least one of the coins.

A Royal Mint spokeswoman says while the face value of the coin is 50 pence, "what it is worth to collectors is another matter".

Rarest commemorative coin

The coin has the rarest commemorative design to be placed into general UK circulation. By comparison, there are about 22.7 million coins bearing part of the shield of the Royal Arms, while there are 7.5 million Girlguiding 50p coins.

If you find one, let us know in the forum thread at the bottom of this story. You can also share a photo of your 50p on The Royal Mint Facebook page, or post it on Twitter using the hashtag #strikingstories.

Shane Bissett, director of commemorative coin at the Royal Mint, says: "Now that people know just how rare this particular coin is, we expect them to disappear from circulation fast."