The iron age weapon was in a grave found by farmer Paul Jenkins on his 22-acre farm on the tiny Bryher island.

Mr Jenkins made the discovery when the front wheel of his tractor went through a hole as he sprayed crops in his field.

"I thought it was an old well," he said.

The farmer used a torch to explore the hole, and found it was an igloo shaped grave - or "cist" - just over three feet wide, nearly two feet deep and 10 inches high.

An 'exceptional' discovery

Mr Jenkins said as soon as he saw the 34-inch long patterned sword, still in its scabbard, he knew it was "something pretty special".

He contacted the British Museum, which told him how to preserve it.

Museum experts will examine the sword in the next few weeks.

Jeanette Ratcliffe, from the Cornwall Archaeological Unit, described it as an "amazing discovery" and said it could date from the fifth century BC.

Retired English Heritage ancient monuments inspector Sarnia Butcher, who lives in the islands, suggested the sword could date from the first or second century BC.

"No other metal work like this and of this date has been found in the islands," she added.

The sword was found on land owned by the Duchy of Cornwall. Duchy land steward Jeremy Poutlin said he would like the relic to go to the islands' museum.

Museum curator Steve Ottery said the discovery was "exceptional".