Today the wartime slogan “Keep Calm and Carry On” adorns mugs, cushions and tea towels. It is a familiar phrase, spawning hundreds of parodies, yet authentic copies of the original government poster are very rare indeed. Even the Imperial War Museum does not own an example.

This week, as the UK faces its biggest political upheaval in 50 years, an original poster will go up for sale at the Art & Antiques Fair, Olympia for more than £20,000. The scarcity of the genuine artwork stems from its history as an emergency message from the second world war. It was never intended for release unless German air attacks on Britain threatened the nation’s infrastructure or enemy forces mounted an invasion.

The poster was designed by the Ministry of Information in the summer of 1939 to represent a message from the King to his subjects, and it was hoped it would reassure the public and prevent widespread panic. A year later, once Britain had weathered the onslaught of the Blitz, all the printed posters were sent back for pulping and recycling as part of the wider paper salvage drive, due to the shortage of raw materials.

The surviving Keep Calm print will go on sale at the fair in Olympia with a price tag of £21,250 at the Manning Fine Art stand. It was discovered 16 years ago at the bottom of a box of old books by Stuart Manley, the owner of Barter Books in Alnwick, Northumberland. Manley and his wife, Mary, framed it and hung it on the wall behind the cash register. After interest from customers, a few reproductions were made and sold. Since then the poster has become internationally recognised and is widely associated with a belief in British stoicism and the “stiff upper lip”. The first ministry print run produced almost 2,500,000 copies of Keep Calm and Carry On, but until 2012 – when 20 copies turned up on an episode of the BBC’s Antiques Roadshow – it was believed that only two copies had escaped pulping.

The Keep Calm design was the least popular of a series of three Home Publicity posters, each headed with a representation of the Tudor crown as a symbol of the head of state. The others read “Your Courage, Your Cheerfulness, Your Resolution Will Bring Us Victory” and “Freedom Is in Peril. Defend It With All Your Might”. The key words “Your Courage”, suggested by a civil servant named AP Waterfield, were regarded as potentially the most effective as “a rallying war cry that will bring out the best in every one of us and put us in an offensive mood at once”.

The posters were dispatched across the country, to mixed results: Mass Observation reports from the time suggest the tone of even this milder slogan was regarded as patronising.

Draft versions of the three posters were completed on 6 July 1939, and were agreed by the home secretary of the day, Samuel Hoare, in August. They were to be ready to send out within 24 hours of the declaration of war. The typeface is close to Gill Sans but it is suspected the lettering was actually hand drawn.

In August 2011 a British-based company registered the slogan as a trademark in Europe and the United States, after failing to obtain registration of the slogan as a trademark in the United Kingdom. The registration was later queried on the grounds that the words are too widely used for anyone to own exclusive rights, but the request for cancellation was rejected.