The family of Sabina Suey claim a British secret agent approached her in a Valencian street in the late 1930s.
In exchange for the cup he offered Suey and her family safe passage from civil-war-blighted Spain to England.
Suey refused – her family recalled she had “an unbreakable will”.
A trained pianist, she regularly attended Mass in Valencia Cathedral, home since the fifteenth-century to a pearl-inlaid chalice said to have been used by Christ at the Last Supper.
On 20 July 1936, three days after the outbreak of the Spanish Civil War, a canon of Valencia Cathedral asked Suey to hide it.
Dr Ana Mafé García, an expert on the Holy Grail, said: “Hours after Suey removed the grail, the [Republican] militia entered the cathedral, setting pews ablaze and seeking the grail.”
By then, it was hidden in a box wrapped in cotton at the back of a wardrobe in Suey’s home, a three-minute walk from the cathedral.
“From 22 July to 22 August the militia conducted a house-to-house search for the grail,” said Mafé García. “They searched Suey’s home twice, once with a 12-strong armed patrol.”
The pianist and her sister eventually hid the grail at a family property in the country. The story of Suey’s courage and the grail’s eventual post-war recovery is well-known in Spain.
Yet claims of the involvement of British intelligence surfaced only recently, in papers given by Suey’s niece Amparo Donate Suey to Mafé García.
She believes MI6’s counter-espionage activities alerted them to Nazi interest in the Holy Grail.
“From 1935, the Nazis began to search for relics including the Holy Grail. We know Himmler visited Spain,” she said.
The author Jimmy Burns confirmed Hitler and Himmler both believed the Holy Grail was in Spain.
While researching Papa Spy, a book tracking his father’s wartime intelligence activity in Spain, Burns heard the Nazis believed the Holy Grail was held by the monastery of Montserrat near Barcelona.
“The idea was dreamed up by some fanatical Nazi propagandist,” he said. “I asked one of the oldest monks about it and he told me it was all pure fable.”
Although 200 places ranging from Scotland to America say they possess the grail, Valencia’s claim is highly regarded.
Archaeological research has dated the chalice to between the first century BC and the second century AD, while its style suggests Middle Eastern craftsmanship.