Lizzie Norton, 53, who runs the Kensington travel agency Ski  Solutions, is an avid collector of vintage ski posters.

She says: “They must be worth at least £70,000, but our total outlay has not exceeded £20,000 over the years. These pieces of commercial artwork commissioned by the railway and ski resort companies have way out-performed my Isa portfolio.”

Norton collects the posters with her husband, and has seen substantial increases in their value. “One poster of Adelboden that we bought for £200 in 1997 went for £500 in January. Another of Zermatt that cost £380 in 1999, fetched £2,000 in January 2006.

“We now have more than 60 posters, bought over the last 15 years. We keep most of them at our home in London, but we also have some in our apartment in Zermatt, and some in my office. The posters mainly span the period from the 1920s to the 1940s.

“Vintage posters are the ones that make the highest returns and are the best investments. We were lucky, because we got in early, and they have become hugely popular.”

Norton has been in the ski travel business since her early 20s. She and her husband, a travel journalist, have only bought scenes of resorts they have visited.

“We have always stayed at the scene on the poster. However, we never bought them as a clever investment, only because we like them. Now we realise they could be a supplementary pension fund, and a very good legacy.

“We have probably slowed down a bit, though. First, we are running out of space, and the posters have got more expensive, even taking inflation into account.

“Most of them are a fairly standard size, 65 cm by 95 cm, so you need a lot of space. That is one reason why people buy them. They might be living in a contemporary loft with enormous wall space and want a cheap form of art.”

Another group of private buyers are people who have invested in chalets and apartments in the Alps. The owners of such homes want pictures that are relevant to the places they have bought. It is extremely rare to find this type of art in an antique shop.

Norton says: “We always hope we might come across some lying in a dusty corner of an antique shop in the Alps, but their popularity is such now that the owners know their value.” There is one ski poster sale a year at Christie’s, the principal source of the couple’s hoard.

The collector says: “When the auction catalogue comes out we go through it and pick out the pieces we like. A handful of posters come from North America, and there is an online agent selling vintage ski posters.”

The couple bought their first poster in 1992, in Aspen, Colorado for around £300. “We were window-shopping after skiing and spotted the Swissair poster – our favourite airline,” she says.

Norton bought another poster for her godson who is French and lives in Val d’Isère. It has Sports D’Hivers en France written on it and would have been produced in the 1950s. “I gave it to him as a christening present and it cost me £500. He is now six years old and it is worth £2,200.

“I am constantly surprised that this type of popular art can make so much profit. They are not exclusive, as you are not buying an original piece of art. Even the one I bought my godson comes up at auction again and again. It is a fairly abstract picture of skiers going down a mountain.

“There is no way we could have predicted which posters would increase in value. To me, it is still a bit of a lottery.”

The posters that fetch the highest prices are of the swankiest resorts: St Moritz and Villars in Switzerland, and Megève and Chamonix in France. Posters of St Moritz can command in excess of £20,000.

Norton points out: “Sadly for the owners of properties in places such as Verbier in Switzerland, and Val d’Isère and Courchevel in France, these resorts only grew up after the war so they don’t have a history of poster advertising.”

Norton says: “It is difficult to know exactly how much our individual posters are worth. Some of them we have never seen in any other sale. We do try to monitor the ones that we have already, and make a note of the prices they fetch when they reappear on the market. This is also part of the excitement of attending a sale.”

While Norton intends to hold on to her favourite posters into old age, she may let some of them go if she and her husband were to move to a smaller property. “If we don’t need the money in later life we will probably hand down a few posters to younger family members who are keen skiers,” she says.

But one piece Norton would never part with is an iconic image of the Matterhorn. “The reason I love it is because Zermatt is our second home, and has very good memories for us. It is my favourite place in the world. We bought the poster in October 1994 and paid £800. It would definitely be worth far, far more today, and not having come up in any sale since, it has got a rarity value.”