After being salvaged by dozens of conservators, academics and scientists at the Hamilton Kerr Institute in Cambridge, the heartbreaking ruin of a great work of art has been exposed at the National Gallery yesterday.

The revelation has come after 30 years of debate about whether it was too fragile for anything to be attempted.

The Westminster Retable, made almost 750 years ago has now become the oldest surviving altarpiece in England and a masterpiece of European medieval art, described Paul Binski, an expert on medieval art from Cambridge University.

Binski coomented that it was "a miracle of miniaturisation" as it displays no less than the whole world, the sea teeming with fish, the land burgeoning with plants, clouds half hiding a crescent moon and a sun painted as a blazing Catherine wheel.

The painting shows 'astonishing sophistication and quality for this early date', derived from French sculpture, supposedly made by Henry's court painters, as Dr. Binski believes, was almost certainly commissioned for Westminster Abbey by Henry II, who had just remodelled the entire building in the fashionable Gothic style at his own expense, for the dedication of the new high altar in 1269.

From this time, the altarpiece never left Westminster Abbey, during the next seven centuries, but had a very hard life there, surviving dissolution of the monasteries and the iconoclasts of the Reformation. It was even turned into a cupboard lid and in the 18th century it became a display case for the abbey museum to show an effigy of William Pitt the Elder.

In the early 19th century its importance was recognised and the altarpiece was put in a glass case, but it still suffered further from two crusade restoration attempts after having the most damaged paintings removed, and given a coat of green and white painting. Dr. Binski described it as "a victim less of Reformation iconoclasts than of Georgian tourism."

The head of collections at the National Gallery and curator of the exhibition, Susan Foister, yesterday expressed her happiness,"I think it is just the most fantastic thing, we have tragically almost nothing in oil painting on panel from that early medieval period, and this shows a quite outstanding quality which can stand comparison with what is happening anywhere else in Europe."

The altarpiece will remain at the National Gallery until September and afterwards will return to Westminster Abbey for permanent display.