A flower pot at a palatial English estate has been identified as a long-lost Roman sarcophagus worth almost $500,000.

Antique experts visiting Blenheim Palace earlier this week marvelled at the intricate designs on a large casket that has been used to hold flowers on the estate for years.

The officials were originally on the grounds to examine several other historic pieces but dropped what they were doing to assess the makeshift flower pot when they saw it tucked away in a corner.

The experts determined the container was a marble sarcophagus dating back to 300AD.

The archaeological marvel features carved lion's heads and Greek gods including Dionysus, the god of wine, Hercules and Ariadne.