It was an ill-fated vessel, built on the Clyde, which became involved in an abortive gun-running operation for Confederate agents in the American Civil War before sinking off the Devon coast.

Now the Govan-built Iona II is threatened by unauthorised diving and decay, heritage experts have warned.

The ship has been included in a Heritage at Risk (Har) study aimed at helping conservationists identify why such "historic treasures" are in danger of being lost and what can be done to preserve them for future study.

English Heritage is so concerned about its rate of erosion it has put the Iona II in its top 10 protected wreck sites at risk.

The 245ft paddle steamer was built in 1863 by J & G Thompson of Govan to be used as a fast ferry around the Clyde. Its 20ft paddle wheels pushed it along at a rate of 24 knots. English Heritage documents read: "After only one season working as the Clyde ferry, Confederate agents bought the vessel on behalf of Charles Hopkins Boster of Richmond, Virginia."

It left on January 19, 1864, and stopped at Queenstown (now Cobh) near Cork in the Republic of Ireland for coal before crossing the Atlantic.

Hours of pounding by heavy seas caused the vessel to take on water and, after seeking refuge in the Bristol Channel, the Iona II sank in 20 metres of water east of the island of Lundy.

The site was discovered in 1976 when a diving company came across it while looking for the wreck of a merchant ship. Iona II was designated protected in 1990.

Dr Robert Prescott, director of the Scottish Institute of Maritime Studies at St Andrews Unversity, said: "Iona II is a fascinating example of the use of high- speed steam-powered vessels built on the Clyde that were used in the American Civil War as blockade runners. That is what gives it its significance."