Olympians and Paralympians turned out to launch the competition
Viewers of BBC TV's Blue Peter are being given the chance to design a 50p piece to mark the 2012 Olympic and Paralympic Games in London.
The winner of the competition, open to those aged between six and 12, will be the first child to see their work featured on a UK coin in circulation.
The athletics-themed coin will be issued by the Royal Mint in 2010.
The Blue Peter contest is part of a wider Royal Mint competition which will see 29 Olympic designs appear on coins.
Children have also been able to take part in other contests to design coins for the Royal Mint - although none have been winners.
Blue Peter editor Tim Levell said: "This is the first time the Royal Mint has given a child the guaranteed chance to design a circulation coin, so this really is making history."
The winner, who will be notified in October 2009, will also receive a gold version of their design and an invitation to see it being turned into coins at the Royal Mint in Llantrisant.
The 29 coins will each depict a different Olympic or Paralympic sport.
The Blue Peter competition is open to UK residents aged between six and 12 at midnight on the closing date of 2 March 2009.
Entries need to be submitted on an official template which can be downloaded from the programme's website or obtained by post.
Blue Peter presenter Helen Skelton says coin competition is 'making history'
The "tails" side design cannot feature an image of a recognisable real person, whether alive or dead, or the Olympic rings.
The entries will be seen by a panel of judges made up of independent experts, and representatives of the Royal Mint, the BBC, London 2012 and the International Olympics Committee.
They will be split into three age groups and a top entry selected from each. A winning selection will then be made before being passed on to the Royal Mint Advisory Committee, the Treasury and the Queen for formal approval.
The contest is only the second to be run by Blue Peter since 2007 when it was revealed that a poll to name the show's cat had been rigged and programme staff had asked a child visiting the studio to pose as a phone-in competition winner.
Mr Levell said: "We are focusing on fewer but bigger competitions. We want to make sure those that we run are the best, are for the most exciting prizes and that they are run without any problems."
Blue Peter said the entries would be kept securely and entered into a database before being sent to the Royal Mint.
On top of the TV tie-in, 27 designs will be chosen from entries to the Royal Mint from the general public, and a cycling themed design contest will be open to secondary school students aged 13-19 years only.
The student winner will be given £10,000 for their school, plus a gold coin featuring the design. The other winning entrants will receive £1,000 as well as a gold coin.
The competition was launched in Trafalgar Square by former triple jump champion Jonathan Edwards and six other Olympic and Paralympic medal winners - Giles Long, Karen Pickering, Nick Gillingham, Danny Crates, Ian Rose and Anna Hennings.
Edwards, who took gold medal at the Sydney in 2000 said: "This is an amazing opportunity for the whole nation to be part of London 2012 in a lasting, memorable way."
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.