Lord Lloyd Webber bought the painting for �18m in 1995

A US judge is blocking the auction, due to take place at Christie's in New York until the matter is fully investigated.

The piece, Portrait of Angel Fernandez de Soto, taken from the artist's Blue Period, is worth up to $60m (�31.5m).

In a statement the foundation said the claims were "utterly spurious, without legal or factual substance."

Julius H Schoeps, an heir to Berlin banker Paul von Mendelssohn-Bartholdy, claims his ancestor had to sell the painting at a low price after he was forced to flee his mansion during the 1930s.

Mr von Mendelssohn-Bartholdy, whose family had converted from Judaism to Christianity, died in 1935.

Good causes

A foundation spokesman said: "During the eleven years the charity has owned the picture no-one has previously raised any questions about the ownership.

"It has been exhibited publicly several times at the National Gallery as well as the Royal Academy of Arts in London and the New York Fine Arts Fair," he added.

The foundation also claims it only found out about the legal action two days before the auction was due to take place.

Christie's declined to comment on the case.

Lord Lloyd Webber bought the painting in 1995 for �18m and at the time he said the artwork was "mesmeric".