The Nuestra Senora de las Mercedes, was sunk by the British navy southwest of Portugal in 1804 while sailing back from South America with more than 200 people on board. A US court ruled last year the loot belongs to Spain but the company has appealed and is still holding the treasure in the US.

So far, 15 of the new sites have been analysed and the only thing of value that has turned up is an 18th-century anchor, the newspaper El Pais reported.

It quoted Navy Admiral Daniel Gonzalez-Aller, the director of the search effort, as saying most of the sites examined so far will be ruled out as worthless and include remains of seaside human settlements and possibly even junk like washing machines.

Carme Chacon and Angeles Gonzalez-Sinde, Spain's defence and culture ministers, boarded one of the minesweepers taking part in the search and insisted they are very serious about protecting vestiges of Spain's past.

"Where some see loot, we see our history. Where some look for gold, we find our heritage. Where others would seek to pillage, our calling is to conserve," Ms Chacon said in a speech aboard the ship.

The Culture Ministry estimates there are more than 3,000 sites in Spanish coastal waters with shipwrecks, remains of aircraft, submarines or human settlements, but most of them are remains of ships. Of that total, it says as many as 800 could be in waters off Cadiz.

Future stages of the search campaign will target other areas of Spain's coast, with the ultimate goal of developing a map of where on the seabed shipwrecks lie.