It's the pin-up of our age, gracing homes, shops - even a US embassy. Jon Henley on the poster we just can't stop buying

Nowadays, of course, it would be farmed out to an expensive communications agency. Back in the spring of 1939, it was an anonymous civil servant who was entrusted with finding the slogan for a propaganda poster intended to comfort and inspire the populace should, heaven forbid, the massed armies of Nazi Germany ever cross the Channel.

This was the third in a series. The first, designed to stiffen public resolve ahead of likely gas attacks and bombing raids, was printed in a run of more than a million and read: Your Courage, Your Cheerfulness, Your Resolution Will Bring Us Victory. The second, identically styled, stated: Freedom Is In Peril.

From August 1939, both posters began appearing all over the country, on billboards, in shops, on railway platforms. The third, though, was held back. This one was for the real crisis: invasion. A few may have made their way on to select officials' walls, but the vast majority of the British public never got to see it. This poster enjoined: Keep Calm And Carry On.

And suddenly these days, it's everywhere, from homes to pubs to government offices. The Lord Chamberlain's Office at Buckingham Palace, the prime minister's strategy unit at No 10, the Serious Fraud Office, the US embassy in Belgium, the vice chancellor of Cambridge University, the Emergency Planning Office at Nottingham council and the officers' mess in Basra have all ordered posters. Even David Beckham has the T-shirt, we are told.

For 60 years, the poster had been forgotten. Then, one day in 2000, Stuart Manley, co-owner with his wife Mary of Barter Books in Alnwick, Northumberland, was sifting through a box of hardbacks he had bought at auction when he saw "A big piece of paper folded up at the bottom. I opened it out, and I thought, wow. That's quite something. I showed it to Mary, and she agreed. So we framed it and put it up on the bookshop wall. And that's where it all started."

Today, you can buy Keep Calm and Carry On mugs, doormats, T-shirts, hoodies, cufflinks, baby clothes and flight bags from any number of retailers. You can use the design as a screensaver for your computer or mobile phone. There are even spoofs. Facsimiles of the poster itself, which Barter Books initially reproduced after a rash of customers asked to buy its copy (one offered £1,000), have sold in their tens of thousands.

Manley has sold some 41,000 posters. He is not the only one. Mike Coop of keepcalmandcarryon.com reckons he is shifting 300-500 products a week (and admits to having tried to trademark the phrase); the Keep Calm Gallery's Lucas Lepola is selling "probably around 500 a month". Keep Calm and Carry On may not yet be in the same league as Athena's all-time poster hits, Tennis Girl (2m sales) and Man and Baby (5m), but it is clearly striking a chord. The reasons, experts reckon, are manifold.

Alain Samson, a social psychologist at the London School of Economics, says that in times of difficulty, "people are brought together by looking for common values or purposes, symbolised by the crown and the message of resilience. The words are also particularly positive, reassuring, in a period of uncertainty, anxiety, even perhaps of cynicism."

Dr Lesley Prince, who lectures in social psychology at Birmingham University, is blunter still. "It is a quiet, calm, authoritative, no-bullshit voice of reason," he says. "It's not about British stiff upper lip, really. The point is that people have been sold a lie since the 1970s. They were promised the earth and now they're worried about everything - their jobs, their homes, their bank, their money, their pension. This is saying, look, somebody out there knows what's going on, and it'll be all right".