Is the Psaier legend a hoax, or did he really exist? Did he die in the 2004 Boxing Day tsunami, and was he so closely associated with Pop artist Andy Warhol that he could have been responsible for creating a significant tranche of his work?
These are the questions at the centre of the dispute between Mr Nicholson, who has conducted sales of Psaier's work, and the Warhol Foundation, who guard the Andy Warhol estate.
In The Times on August 7, the Foundation and the Warhol Museum dismissed any connection between Psaier and Warhol, saying there was no mention of him in their archives. Foundation board member and former Warhol associate Vincent Fremont, who was at the Factory when Psaier was said to have been there, was quoted as saying he had never heard of him.
But in addition to the dozens - possibly hundreds - of works he has in storage, Mr Nicholson has a wealth of papers, documents and photographs that together create a seemingly credible history, pointing not only to the existence of Pietro Psaier, but also to his close association with Warhol.
He further argues that Psaier's name may never have come up at the Factory because he would have been known there as 'Peter the Italian'.
Together with Jacqueline Chapman, who has spent some considerable time researching the life and work of Psaier, he is now determined to prove his point to the satisfaction of the art world and publish the catalogue raisonné.
And he says he also has no intention of changing his plans for the next major sale of Psaier's work at his Fernhurst rooms on September 17.
Hoax or not, Psaier is certainly an enigma. Although there are several references to people who directly knew him, and at least two individuals who saw him and Warhol together, attempts to contact them have so far not borne fruit. And there are undoubted inconsistencies with dates - was he born in 1936 or 1939? - and places.
Nevertheless, the sheer weight of material to support the legend warrants extremely serious consideration.
Warholstars, the website dedicated to Andy Warhol's life and work, have been outspoken in their criticism of John Nicholson and have already dismissed the whole Psaier story as a scam to exploit the Warhol name.
Their position appears unchanged, but they now appear to be taking the matter more seriously and have published a 12-page critique of Nicholson's web and catalogue content on the artist as well as information from other sources.
ATG - the only publication to inspect the Nicholson archive so far - are following a number of leads that have emerged as a result of visiting John Nicholson's to see the art and documents. Here is a sample of what we have found:
• A large number of works at Nicholson's that are impressed with what appears to be a Factory verification stamp confirming them as joint works between Warhol and Psaier.
• A large number of additional works, not linked to Warhol, including architectural drawings and designs for cars, all signed and some dated as early as the 1950s.
• An interview with Psaier, dated February 25, 1990, from the now defunct Madrid newspaper El Independiente Jovenes, complete with a photograph of the artist.
• A photo album of the artist at work - probably in the 1980s - with the name Pietro Psaier clearly visible in the background as a 3-D logo mounted on the wall. The album also contains photos dating back to the early 1990s of banners being erected around Marbella, including on a bridge, publicising a Psaier exhibition.
• A leaflet for a private view to a Psaier exhibition at the Hotel Ritz Madrid, including photographs of, and an interview with, the artist and details of his links to the Factory and even John Lennon. One of the photos is attributed to a 1993 issue of Downtown magazine, dating the exhibition as later, although it is the same photo that appears in the 1990 El Independiente Jovenes article.
• Numerous independent print and web references to auction lots sold, many through Madrid auctioneer Fernando Duran, and exhibitions held - including at least one joint Warhol/Psaier exhibition that is still taking place in Vigo - dating back to at least the 1990s if not earlier.
If the enormous body of material seen by ATG is part of a hoax, then it would appear to be on an industrial scale stretching back decades and across several continents.
The evidence appears compelling to show that there was an artist called Pietro Psaier, who created a vast body of work. There also appears to be significant evidence of a link to Warhol and the Factory, with Psaier contributing to Warhol's work, although absolute proof of this continues to evade those looking for it.
By Ivan Macquisten