LONDON (Reuters) - A specialist firm has offered to buy faulty 20p pieces for 50 pounds, sending Britons scrambling in search of one of the approximately 50,000 undated coins.

However, other coin dealers have advised people to hold on to them because they may be worth as much as 300 pounds within the next decade.

Specialist coin company the London Mint Office made its offer for 20p pieces that had been cast with a new tails design, but with the old head design, resulting in a batch of coins with no date on them.

The letters "F.D." have been printed where the date "2008" should have been.

A coin with mismatched sides is known as a "mule" and they are extremely rare. The last time one appeared in circulation was during the reign of Charles II more than 300 years ago.

The coins first appeared at the end of 2008, after a batch which included between 50,000 and 200,000 of the faulty 20ps entered circulation.

The last time a mistake was made on the casting of a coin was in 1983, when a batch of 2p coins went into circulation with the words "New Pence" rather than "Two Pence" on the reverse side. These coins fetched up to 650 pounds in top condition.

The Royal Mint, which produces Britain's coin, has stated that the coins are legal tender, but has not made any further comment on their current or potential value.

(Reporting by Nat Arkwright; Editing by Stefano Ambrogi)