Two-and-a-half million copies of "Keep Calm" were printed, to be distributed in the event of a national catastrophe, but remained in storage throughout the war.
The message was all but forgotten until 2000, when a copy was discovered in a box of books bought at auction by Stuart Manley, a bookseller from Northumberland.
"I didn't know anything about it but I showed it to my wife. We both liked it so we decided to frame it and put it in the shop," explains Mr Manley.
"Lots of people saw it and wanted to buy it. We refused all offers but eventually we decided we should get copies made for sale."
Sales remained modest until 2005, when it was featured as a Christmas gift idea in a national newspaper supplement.
"All hell broke loose," says Mr Manley.
"Our website broke down under the strain, the phone never stopped ringing and virtually every member of staff had to be diverted into packing posters."
Soothing words
The poster was just one of hundreds produced by the Ministry of Information during the war to influence public opinion.
"The poster was a major medium in a way that it isn't now," says Professor Jim Aulich, an expert in propaganda art at Manchester Metropolitan University.
"It wasn't competing with television. It was one of the main ways of reaching people, through billboards and on public transport."
Rescued from obscurity after 70 years, the Ministry of Information's appeal for calm has risen to cult status. Mr Manley's store, Barter Books in Alnwick, receives an average of 1,000 orders a month from around the world. Customers include 10 Downing Street and assorted embassies. The design has been reproduced on T-shirts and coffee mugs, shopping bags and cufflinks.
It has also spawned imitators. One company has given it a twist, replacing the original slogan with "Now Panic and Freak Out".
To some, the world in 2009 seems as uncertain as it was in 1939, even if modern-day anxieties focus on redundancy and recession rather than bombs and the Blitz. Perhaps this is why the message still seems so relevant.
Of course, it might be difficult for the current government to come up with a poster with quite the same appeal during this time of economic stress. Context is everything, says social psychologist Dr Lesley Prince.
"If the government is in tune with you, you will listen. If you think the government is working on your behalf, you will listen."
This was indisputably the case during WWII, but is less clear-cut even in the most troubled period of peacetime.
Stiff upper lip
And a message of such powerful simplicity might not be so forthcoming these days. Today's government posters attempt to convince the public of an unappreciated danger and get them to modify their behaviour. The "Keep Calm" poster is merely an injunction to think another way and continue acting as you have always acted.
"It's very good, almost zen," says Dr Prince. "It works as a personal mantra now. If people are thinking 'I'm about to lose the house', it's good advice."
People are drawn to the calming Britishness of the message, says Mr Manley.
"It's interesting to look at the kind of places we often sell to; doctors' surgeries, hospitals, schools and government departments. It seems to strike a chord anywhere that works at a hectic pace."
Prof Aulich adds that the message has universal appeal.
"It speaks to peoples' personal neuroses. It's not ideological, it's not urging people to fight for freedom like some propaganda posters did."
Following the end of WWII, most of the posters are believed to have been pulped, never having seen the light of day. Only two original copies are known to have survived.
Thanks to a chance discovery in a dusty box of books, the soothing entreaty is finally having its intended effect, bringing comfort to a nation in turmoil.
