The pounds 5m canvas, entitled simply The Waterlilies, has been on display in the same exhibition during its stay in the US, at Boston's Museum of Fine Arts. But it is now thought to rightfully belong to descendants of the Jewish art dealer Paul Rosenberg, who want it back.
Research carried out by the London-based Art Loss Register has indicated that the 35in by 36in painting was confiscated by the Nazis in 1941. It is then believed to have joined the body of plundered art treasures put together for von Ribbentrop - the man who organised the systematic theft of Jewish-owned art from occupied countries.
The Rosenberg family are currently considering what action, if any, to take before the painting is due to appear in the London gallery. A lawsuit which prevents the work leaving the US is not out of the question.
The sumptuous work is one of 48 depictions of the lilies that bloomed in Monet's garden at Giverny, France. This canvas, which was tracked down from a family photograph, is just one of 58 works which the Rosenbergs, who live in New York, asked the Art Loss Register to trace. Since 1975 the painting has been in the care of the Musee des Beaux-Arts at Caen in Normandy and has been held in trust by the Musees Nationaux de France since 1950.
The RA said that both they and Boston had been aware that the painting was one of a group which had been recovered after the war, and that the provenance given in the exhibition reflected that. David Gordon, secretary of the RA, said: "We are very pleased, if this exhibition has helped in any way, to restore great works of art."
James Emson, director of the Art Loss Register, stresses its investigations were under way before it was known the painting would form part of the exhibition. "It is important to remember that this is a very shady area of history," he said.
"We do know, however, that this painting was in the collection of von Ribbentrop and was among 40,000 items seized by the Allies and handed over to France at the end of the war. All bar 2,058 were returned to their owners and the remainder were distributed for safe-keeping to Paris and provincial museums."
What has suddenly made it feasible to pinpoint such art crimes is the compilation in London of a Jewish master list of all works taken from owners in occupied Europe. The list now numbers more than 3,000 picture entries and, with backing from art dealers and auction houses, it is thought 250,000 items will eventually appear. Listed works will immediately be taken off the market unless the former owners issue an official "reprieve".