Brits have been told they could be sitting on a small fortune in the form of valuable coins in their change.

Coin collecting is a very popular hobby, and coins often sell for hundreds or even thousands of times their face value at auctions and on sites such as eBay.

One Royal Mint coin that is particularly sought after is the 250th anniversary Kew Gardens 50p coin.

The coin features the Chinese Pagoda at the Royal Botanic Gardens on the tails side and a portrait of the Queen.

But how can you tell if you have a valuable coin? Mat Biernacki, Coins & Currency Specialist at Vintage Cash Cow, explained what to look out for.

How to spot a valuable rare coin

1. It has a low mintage number (it’s rare)

You’ve probably heard about mintage in relation to coins; and quite rightly, as much of a coin’s value depends on its mintage. But what does it actually mean?

Put simply, a coin’s mintage is the number of coins that have been produced. For example, if the mintage number is 100,000, only 100,000 of the specific coin have been produced. This means that the lower the mintage, the rarer the coin. And – you guessed it – the rarer a coin, the more likely it is to be valuable.

Low mintage can be deliberate, such as with limited edition coins, which are purposely made to be scarce. However, it can also occur due to ageing, as coins can originally be issued in the thousands or millions, but are gradually lost, worn down and therefore become rare.

If you have an unfamiliar British coin that you think might be valuable, you can find the mintage number on Royal Mint’s coin archive.

2. It was struck with an error

Very few coins with ‘minting’ errors actually make it into circulation, as they’re spotted at the mint’s quality control before they make it into circulation. This means that the error coins that do make it out are often incredibly rare – so if you have one in your collection, it could be worth a small fortune.

For example, a coin with a rotated die – when the reverse of one side of the coin isn’t perfectly aligned with the other – could be extremely valuable. The design on the coin should be the same way up as the Queen's head when the coin is turned over. The more rotated the other side, the more valuable the coin could be.

Another valuable error is when a coin is struck off centre and is therefore missing part of the design. These coins can be highly valuable, but it does depend on how far off centre they‘re struck and the specific impact this has had on the design. For example, a missing date or digit due to being struck off centre could actually end up lowering the value.

3. It’s historically significant

Coins that commemorate a significant historical event or period of time are often highly valuable.

This is because they’re typically (though not always) produced in limited quantities, making them rarer. They also have the additional benefits of emotional resonance and nostalgia, which increases demand and subsequently increases their market value.

Hundreds of commemorative coins have been struck in the UK, namely honouring coronations, jubilees or royal deaths, military history, technological advancements, cultural milestones, prominent British figures and even noteworthy social movements – the list goes on and on.