The wraps have come off one of Westminster Abbey's least known treasures, a medieval marble pavement foretelling the end of the world, while conservation experts consider how to preserve the ancient stones for the next 740 years.

Few modern visitors have ever seen it, although since 1268 kings and princes, queens and cardinals have walked across a symbol laden mosaic as intricate as a piece of jewellery.

It is made up of rare marbles and gemstones, including some recycled from monuments 1,000 years older, and pieces of coloured glass, set in complex allegorical patterns into a framework of Purbeck marble cut as intricately as a jigsaw puzzle.

"When this floor was new it would have blazed with colour," Vanessa Simeoni, the abbey's head of conservation said. "The materials were chosen for their brilliance and shine, and the quality of the craftsmanship is actually shocking, the ultimate that could be achieved."

The mosaics are known as Cosmati work, after the four generations of a Roman family of marble workers who perfected the technique. The Westminster one, regarded as the finest north of the Alps, uniquely has an inscription boasting of its makers - and a cryptic message about the end of the world.

It was laid in the 1260s, when Henry III sent his new Abbot of Westminster, Richard de Ware, for talks with the Pope in Rome. The Englishman saw a newly installed pavement in the Pope's summer residence, knew it was just the thing for the cathedral which Henry was spectacularly rebuilding around the tomb of St Edward the Confessor, and arrived home with a ship load of marble, glass and Italian craftsmen. Ware's reward was his own tomb incorporated into the design. Henry's tomb, and the saint's shrine, were originally covered in similar work, but all the scraps of marble and glass were picked out as sacred relics by generations of pilgrims.

Only a handful of brass letters remains of the original long inscription, but it was transcribed centuries ago. It names the king, the chief craftsman as Odoricus, gives the date in a tortuous riddle, and then mysteriously suggests that the world will last for 19,683 years, by adding together the life spans of different animals: "add dogs and horses and men, stags and ravens, eagles, enormous whales ...."

Careful cleaning, and a radar survey has revealed that although the pavement bears the scars of centuries of repairs and patching, crude and careful, most of it is original, the rich green and plum-coloured porphyry - almost certainly from chopped up ancient Roman sculptures and architectural fragments - still bedded in the limestone mortar laid by the medieval craftsmen.

For most of the past 150 years it has been covered in thick layers of carpet intended to protect but in fact just adding to the dirt and staining.

Even when the Queen was crowned above it in 1953, the royal pavement was covered over.

The two-year restoration programme will now stabilise the pavement, so that a treasure from the middle ages can be permanently displayed in a 21st century cathedral.