An inscription etched on a marble tablet at a stately home could be a hidden message from an 18th Century Christian sect, code-breakers say.
Mr and Mrs Lawn have different theories about the code
Specialists from Bletchley Park in Buckinghamshire were asked to decipher the inscription on the Shepherd's Monument at Shugborough, Staffordshire.
The code has baffled great minds for years and had been rumoured to point to the location of the Holy Grail.
Experts now think the code is a message from a sect called the Priory of Sion.
The encoded ornament is located in the grounds of the ancestral home of the Earls of Lichfield.
It was commissioned in 1748 by the then earl, Thomas Anson, and features a carved image of a Nicholas Poussin painting with the letters D OUOSVAVV M inscribed below.
Poussin was believed to be a Grand Master of the Knights Templar, an order which captured Jerusalem during the Crusades and were known as the keepers of the Holy Grail.
The grail is the cup Jesus is said to have used during the Last Supper and which caught some of his blood during the Crucifixion.
But the code-breakers revealed on Thursday that they believe the cipher is a message from the obscure sect, which is likely to stand for "Jesus (As Deity) Defy".
The message is being interpreted as a declaration of the sect's belief that Jesus Christ was an earthly prophet, not a divinity.
The inscription on the Shepherd's Monument reads D OUOSVAVV M
The order had to keep its views secret because the Church of England thought it was heretical.
The work was led by Oliver and Sheila Lawn, a Sheffield couple in their 80s who were based at the code-breaking centre during World War II.
But while Mr Lawn favoured the Priory of Sion interpretation, Mrs Lawn told the Daily Telegraph that she favoured another theory.
It suggests the eight central letters are code for a Latin poem to a departed love one.
The poem "Optima Uxoris Optima Sororis Viduus Amantissimus Vovit Virtutibus", translates as "Best wife, Best sister, Widower most loving vows virtuously".
"This appears to be an elegant solution," Mrs Lawn told the Telegraph.
The present Earl, Lord Patrick Lichfield said the "love poem" theory was also favoured by his grandmother.
"If it creates the answer to an age-old mystery the whole world might get excited. At the moment, though, I'm quite content with the old theory," he told the BBC.
Richard Kemp from the Shugborough Estate said the inscription proved another link between the monument and the Holy Grail.
"This monument is the second piece of evidence that there is a connection," he said.
"All we knew until now is the Anson family chose to depict in their ground a picture by Poussin.
"Poussin is a grandmaster of the Knights Templar so there was a sort of connection there - Knights Templar, Holy Grail.
"What this says is, the Ansons also put down the bottom of their painting basically a 'Sun' headline of what the Knights Templar stood for.
"In other words that Jesus was not a celestial prophet but an earthly one, which is a racy thing to say."
Mr Lawn said on Thursday that trying to solve the Shugborough code had been "much more difficult" than cracking Enigma.
The work at Bletchley is credited with shortening Word War II
"For any code, you need a minimum amount of encoded material, very much larger than ten letters.
"No code of ten letters is possible to break definitively so to break the Shugborough code, you have to take into account the circumstances and history."
The painting from which the garden carving is drawn, Les Bergers d'Arcadie, is housed in the Louvre in Paris and has been subject to speculation over its possible Masonic symbolism.
Many people have tried to crack the code, including the creator of the theory of evolution, Charles Darwin.