The story of Western civilised art begins with icons - still the only historic art form disdained by the art establishment's museums and galleries. Consequently, they are still the cheapest. John Windsor goes on a pilgrimage.

If you invest in icons hoping that an art-historical penny is about to drop - as, indeed, it should - you could be in for a wait. The fear of fakes and smuggled goods and the centuries-old suspicion that icons are idolatrous will ensure that those radiant saints, even Christ himself, will continue to be excommunicated from the commercial mainstream.

Console yourself by scrutinising the 13th century Italian primitives by Duccio of Sienna, and by Cimabue and Giotto of Florence, in the National Gallery. Are these really the first modern paintings, as the authorities would have us believe?

True, the first, faltering attempts at natural realism, especially the Florentines', presage the Renaissance. But the gold ground, simple forms and strong egg-tempera colours owe inspiration to a magnificent, older tradition of art - icon painting - that has been stuffed under the academic carpet.

Its centre was not in Italy but in Constantinople, today's Istanbul - glittering, ancient citadel of the Byzantine empire and of the Eastern Orthodox Church. There never was a Dark Age in Constantinople. Yet icons painted there by Christian monks before and during the Renaissance are today worth only a fifth or even a tenth of the price they might fetch if they had been painted in Italy.

It is partly a matter of taste. Following the schism between Eastern Orthodox and Western (Roman Catholic) churches in 1054, the Eastern tradition of icon painting, with its archaic, elongated figures and stylised facial expressions, remained intact wherever the Orthodox Church held sway - notably in Russia. By contrast, in Italy, the Renaissance led painting into the naturalism that Western European eyes have become accustomed to.

Rich 18th century Englishmen on Grand Tours bought art in Italy. Russia was a closed country and Constantinople had been sacked by Moslem Turks in 1453. So icons remained, as it were, off the map.

Today, icons are a narrow and unpredictable market, with too few buyers and sellers to establish a track record of prices. Recent scandals have not helped. Prices are still recovering from the fiasco of 1980, when Christie's sold the American George Hahn's collection of Russian icons for record sums, only to have most of them denounced as fakes by a former restorer at the Tretyakov Gallery in Moscow, which had sold them in the 1920s. In the end, the icons turned out to be genuine - but prices had plumetted and stayed low.

Dick Temple, a dealer in icons for 40 years, is optimistic. He says: "It's an accident of history that the West knows more about Asian or American art than Byzantine. But everybody is fascinated by icons and everybody knows that, eventually, they are going to recover.

"The art establishment treats them rather like doctors treat alternative medicine. They know it's there, but they wish it would go away".

Sotheby's got pounds 232,500 two years ago for a 15th century Russian icon - a world record for an icon but still a trifle compared with the millions that Italian Old Masters can fetch. Mr Temple had sold the icon 20 years previously for pounds 12,000. Its title: Christ Among the Doctors.

An exhibition of Russian icons, "Holy Russia", opens at the Royal Academy in London on 19 March. But you can test your eye for them before then by previewing Christie's sale on Thursday (2pm) of the Provatoroff collection and the Russian icons in the general sale that precedes it (10.30am).

Remember that the artists were not striving for naturalistic representation. Icon simply means image. They aimed to project the spirituality of saints and position them in the macrocosm. The stylised outlines - such as the bending in unison of the three myrrh-bearing women at the deposition of Christ - were a convention strictly adhered to through the centuries. Readily identified, they had a narrative impact even on the illiterate - as well as being objects of veneration.

St Nicholas the Wonder Worker, in an 18th century 13in by 11in icon estimated pounds 1,500-pounds 2,000 in the general sale, can always be identified as a bishop with a short beard, receding hair, one hand raised and the other holding the gospels.

Mr Temple offers larger icons than most in the Christie's sale. His 16th century Russian Christ in Majesty, 67in high by 45in wide, is priced pounds 95,000. He has noticed more buyers willing to pay pounds 50,000-pounds 100,000. But he says quality can still be found in the pounds 750-pounds 2,000 range - that is, good painting, good colour, good composition. These are Old Masters, however anonymous.

Icons are still being painted. Long & Ryle Art International sells the work of a brilliant Russian artist, 35-year-old Sergei Fedorov. His effulgent 3ft by 1ft 10in Annunciation in traditional tempera, gold and silver on wood, is priced pounds 4,000.

Maria Andipa's Icon Gallery has decorative 18th century Greek provincial icon fragments for as little as pounds 70. And for coloured reproductions from pounds 2-pounds 4.95 and a calendar of 24 icons in colour, with commentaries (pounds 14.95 plus pounds 5.50 p&p), try St Paul Multimedia run by the Daughters of St Paul. These days, even the Western Church is catching up on icons.

"A Brief Illustrated History of Icons" by Richard Temple, pounds 5 plus pounds 1 p&p: Temple Gallery, 6 Clarendon Cross, London W11 (0171-727 3809). Christie's (0171-839 9060). Royal Academy (0171-300 8000). Long & Ryle Art International, 4 John Islip Street, London SW1 (0171-834 1434). Maria Andipa, 162 Walton Street, London SW3 (0171-589 2317). St Paul Multimedia, 199 High Street, Kensington, London W8 (0171-937 9591).