Roy Davies, a former lorry driver from Wolverhampton, was arrested when the drugs and money were being loaded into a vehicle outside Benidorm in May 1997.
Police found old documents, manuscripts and other artefacts hidden in Davies's luxury villa nearby. The trove includes Papal Bulls from the 17th and 18th centuries and seals from Cromwell's Commonwealth of the 1650s. The haul was revealed this week when the investigating judge Baltasar Garzon lifted the condition of secrecy after the case was referred to Madrid's national court, which deals with drug trafficking. Davies is in prison in Alicante awaiting trial.
Benidorm's police chief, Antonio Perez, said: "All the signs are that the drug ring waskeeping these documents with the intention of selling them at high prices either to collectors or in money-laundering operations to fund other drug deals."
He said the treasures may have been stolen two years ago from a British collector, but there was no firm evidence supporting that theory.
"I cannot say what they are worth," he added. "In one sense they are priceless, particularly to historians."
The hoard was in a wall safe at Davies's villa in the town of L'Alfas del Pi, north of Benidorm. The raid was part of an operation codenamed El Manco - Spanish for "one-armed". Davies, who has lived in the area for nine years, has only one arm.
He also remains a suspect in an unresolved mafia-style killing of Jan Juri Slavinski, his former partner in a television equipment business.
Slavinski, 35, from a circus family in Hamilton, Scotland, was stabbed in a Benidorm villa in August 1996. Davies claims video footage proves he was in a Benidorm nightclub at the time of themurder.
The trove is being held in the vaults of a Benidorm bank until the national court decides what to do with it - or someone claims ownership.
There is no doubt of the authenticity of the two Napoleon letters. One was written in 1813 to his adopted son Eugene, son of his wife Josephine de Beauharnais from her previous marriage.
The English translation reads: "In Magdeburg there are 500 gendarmes ready to leave for Berlin where they will be supplied by the police of that great city", and concludes, "Your affectionate father". The other document was written in Dresden on 26 September 1813.
Handwriting experts are reported to have described Napoleon's signature as "baroque and grandiloquent, out of proportion to the rest of the letter - denoting his megalomania".
The Papal Bulls were signed by Pope Clemente IX in 1668 and Pope Benedict XIV in 1754. They are handwritten in Latin, and they refer to papal pardons, one of them granted to an Italian descendent of the Borgia family.Reuse content