The new medieval gallery at the British Museum is full of beautiful images of saints in ivory, stone, gold and wood – but invisible to visitors, it also holds the bones of 39 real saints, whose discovery came as a shock to their curator.

The relics, packed in tiny bundles of cloth including one scrap of fabric over 1,000 years old, were found when a 12th-century German portable altar was opened for the first time since it came into the British Museum collection in 1902.

It was in for a condition check and cleaning, before going on display in the gallery that opens tomorrow – but to the amazement of James Robinson, curator of medieval antiquities, when it was opened a linen cloth was revealed, and inside it dozens of tiny bundles of cloth, each neatly labelled on little pieces of vellum.

The most precious was the relic of St Benedict, an Italian who in the early 6th century was credited as the father of the western monastic tradition, founding monasteries and establishing guiding principles still followed at many monasteries.