Karen Bowerman: "There are three shirts, but which are genuine?"

The discovery coincides with mounting controversy over the real owner of the shirt the same king wore when he was beheaded in 1649, with three different institutions claiming they own the genuine article.

Details of the wreck are due to be released on Friday - a day before the 350th anniversary of the execution of Charles I at Whitehall in London.

Bloodstain: A detail of the Museum of London's shirt

If the ship is a barge called the Blessing of Burntisland it could rank as the most important British archaeological discovery since Henry VIII's flagship, the Tudor warship Mary Rose, was found in the 1980s.

The treasure on board the vessel, which was sailing for England after Charles I's Scottish coronation, is believed to have included a 280-piece silver dinner service commissioned by Henry VIII.

The search - a joint venture between the Royal Navy and the Burntisland Heritage Trust - began in 1997 and has produced a detailed computer profile of the site.

Divers also visited the wreck in December as part of the verification process.

Naval experts and marine archaeologists are unable to say if the wreck definitely is the Burntisland, but there is "growing optimism" that the discovery will be confirmed as the king's ship as early as this summer.

Forgeries

Meanwhile, as one mystery concerning Charles I nears a solution, another, namely the war of words over who owns his death shirt, continues unabated.

The Museum of London has one shirt, complete with stains that are possibly blood, which has been inconclusively examined by Scotland Yard forensics experts.

A museum spokeswoman said: "When it comes to the textiles, it really is a most exquisite piece and it's the sort of quality you would really expect a king to have had."

Another version is kept in Sussex by ancestors of John Ashburnham, Charles I's personal servant. Their collection also includes letters belonging to the king, the death shroud that covered his body and articles of his underwear.

A third version is the property of the Queen, but it was recently discredited by experts and will not appear in public exhibitions commemorating the king's death.

After his death, royalists declared Charles I a saint and a martyr, thereby placing a premium on his relics and greatly increasing the likelihood of forgeries.