Jonathan Swale, who works for Scottish Natural Heritage, used a laser range-finder at the Calders Geo sea cave at Esha Ness.
He said he found it to be 60ft tall and spanning a floor area of 5,600 square metres.
This, he claims, makes it one-and-a-half times as big as the famous Cheddar Gorge in Somerset.
The geologist, who made the discovery while kayaking, said the crown for the largest cave in the country now seemed to belong to Shetland.
Mr Swale told BBC Scotland of his laser range-finder study: “This is possibly the biggest cave chamber in Britain and that’s what tempted me to come back.”