Imagine opening a dusty suitcase in the attic of your new home and finding inside some 50 neatly folded travel posters mainly from the 1930s and subsequently discovering that each of them is worth at least £300 (€380.92).

That is what happened to a couple when they moved into a house near Cheltenham in England. Many of the posters discovered were of Irish interest and one of them alone is expected to fetch between £800 and £1,200 sterling (€1,218-€1,826) at an auction at Christie's in London on May 27th.

Ms Nicolette White, poster specialist at Christie's, says: "Honestly, I'd never seen as many. An old suitcase and all these posters folded up inside." They were in good condition because they'd been folded and kept safely for decades. They form part of the collection of Irish and Scottish posters in the forthcoming auction, while more will go under the hammer next September.

Irish travel posters from the first half of this century are keenly sought after by collectors, she says. Mind you, they're not all travel posters. "There's two lots of Guinness posters, with two in each lot," she says, and it's expected that each lot will make £400 to £600 sterling. Dating from 1933 and 1934, they measure 30 by 20 inches and were painted by artist John Gilroy. One of them is that of a man holding up the girder with ease with the motto: "Guinness for strength".

The posters in the collection are lithographic posters, that is, the artist does his design on paper and then it is engraved on stone, Ms White says. You can recognise lithographic posters because the paper is much older. And if you look carefully at a modern poster "you can see very, very tiny little dots, while with a lithograph you can actually see the blobs of colour on it", she adds.

With posters from around the 1930s, "we never know exactly how many were printed", she says. But if several thousand were produced, most of these would be displayed for instance in railway stations where they were ripped down in due course. But a certain number were kept aside, sometimes by printers or by the commissioning company. "Some people actually collected them at the time as works of art", while the travel ones often emerge from collections by station masters or other railway employees who may have kept copies back every time they put one up, Ms White says. But the posters that are now worth several hundred pounds are the ordinary versions that were put up on walls in the 1930s. "They are multiples. It is not just one poster," she says. Indeed, they rarely sell the artwork for a poster. "Collectors want the actual poster. I know it sounds strange but that's just the way the market works."

One of the Irish travel posters in the May auction is a 1930 lithographic poster in colours by Norman Wilkinson (1878-1971) entitled: "Ireland for Holidays, Kingstown (Dun Laoghaire), LMS (the London, Midland and Scottish Railway)." Estimated at £800 to £1,200 sterling, it reads: "First class seaside resort and touring centre. The gateway to the beauties of Ireland. Numerous holiday attractions." It is perhaps the most attractive Irish poster in the sale, done in a simple style.

Others include: "Blarney Castle, Co. Cork, Great Southern RYS. (Railways)", circa 1930, which reads: "Ireland Land of Romance". (Estimate: £350-£450.) A 1930 lithograph in colours by Paul Henry (1876-1958) entitled "Come to Ulster for a Better Holiday" is estimated at £500-£800.

Christie's auction of Irish and Scottish posters takes place on May 27th. Readers can contact Ms Nicolette White by telephone: 0044 171 321 3206.