James Martin said at least one of the works was painted on a board produced no earlier than the late 1970s, The New York Times reported. Martin, forensic expert who examined 24 of the 32 paintings at their owner's request, made the comments in a lecture Wednesday sponsored by the International Foundation for Art Research in Manhattan, the newspaper said.
Martin's views further a debate over the origins of the paintings, which were discovered in 2002 in a storage locker in Wainscott, on Long Island. The locker had belonged to the late photographer Herbert Matter, a close friend of the abstract expressionist.
Matter's son Alex discovered the works and attributes them to the artist. But scholars have reached no consensus, despite a range of tests.
Matter's lawyer, Jeremy Epstein, did not immediately respond to a telephone message left at his office early Thursday.
Martin is not the first expert to say that some materials in the paintings postdate Pollock's death. After testing three of the works, researchers at the Harvard University Art Museums said in January that the paints contained pigments not commercially available in the artist's lifetime.
They said the findings did not establish that Pollock had not made the paintings, but strongly suggested they had at least been altered after his death.
Matter said at the time that varnish used in restoring the paintings might have contaminated the Harvard researchers' samples, and he noted that cardboard backing on the works had been dated to Pollock's lifetime.
In the mid-1940s, Pollock made dramatic innovations in a style that would become abstract expressionism. He used pour and drip techniques, manipulated paint with such tools as knives and trowels and at times added sand, broken glass, thumbtacks and other objects to his paint. He died in an Aug. 12, 1956, car crash near his home in Springs, a hamlet near East Hampton on Long Island.