It is thought to have been part of a monastery founded by one of Britain’s ancient rulers and may have been used as a baptistry, for the remains of martyrs or as a royal mauso- leum. Measuring 17m (56ft) in diameter, it lies beneath the staff car park of Herefordshire council in Leominster and came to light during a radar scan of the site.

With the co-operation of English Heritage and the council, archaeologists will begin excavating it next month.

Bruce Watson, a senior archaeologist with the Museum of London Archaeology Service, who will co-ordinate the dig, said: “This is a tremendously important find — an opportunity to rewrite the early history of Christianity.”

John Blair, Fellow and tutor in history at The Queen’s College, Oxford, said: “If it is a late Anglo-Saxon round church, it is of exceptional importance.”

The rotunda is thought to have been part of a monastery founded by Merewalh, ruler of the Magonsaete, the land between the River Severn and the River Wye, after his conversion to Christianity.

Mr Watson said: “A chronicler records that Earl Leofric, who died in 1057, and his wife Countess Godiva — who, according to legend, rode naked through the streets of Coventry — enriched Leominster church with ‘valuable ornaments’. Could the Leominster structure lurking under the cloister car park be part of Leofric’s gift to Leominster’s church?” The structure is part of a site occupied by the medieval priory established by Henry I in 1123 and demolished in the 16th century after Henry VIII’s dissolution of the monasteries.

The rotunda emerged in a geophysical survey by Peter Barker, of Stratascan, a company that specialises in tracing buried remains. The evidence suggests that it is relatively well preserved, unlike those at the Abbey of Bury St Edmunds and Worcester Cathedral.

Mr Watson said: “It means we can look at the interior and discover evidence that relates to its date and function. It should have its contents well preserved. It may be like going to a house and seeing the contents left behind.”

The radar scan suggests the rotunda is full of rubble. “If we’re lucky,” Mr Parker said, “that’ll be the demolished superstructure. That would mean we can reconstruct it.”

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