When we got it to the surface I could not comprehend that something that size existed," said Mr Welch, 45, a doctor in freshwater biology, from Aldershot, Hants. "It was as big as a snooker table."

Mr Welch caught the fish with a large rod normally used for tuna or marlin.

It "bent over double" and the boat moved around in circles as he reeled it in.

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After bringing it to the surface the party towed it to the shore to measure it before releasing it back into the Maeklong River.

The Giant Freshwater Stingray is listed as a vulnerable species on the International Union for Conservation of Nature's red list.

However, a number of recent catches have been reported in rarely explored parts of the river systems in Thailand, prompting anglers to believe the population is thriving.