A backyard stone trough used by a British couple to grow flowers for more 30 years has been identified as an ancient Roman sarcophagus with a six-figure value.

Left behind by previous property owners, the two-metre coffin is covered with elaborate carvings of mythical creatures.

It wasn't until the couple, who have asked to remain anonymous, saw a photo of a similar object in a newspaper that they realised it might be something significant, British newspaper The Times reported.

Specialists have since identified the planter box as the sarcophagus of a wealthy Roman woman, carved from a single piece of Carrara marble and dating back to the first or second century AD.

Initial estimates value the casket at more than STG100,000 ($A151,607)

The owners plan to remove the shrubs and soil before sending the item to auction on February 14.