A painting by the Austrian artist Gustav Klimt that was believed lost for the past 100 years has been sold at auction in Vienna.

The unfinished work, Portrait of Fraulein Lieser, fetched €30m (£26m; $32m).

It was commissioned by a family of Jewish industrialists in 1917, a year before Klimt's death.

However, there are many unanswered questions about the painting and debates about who the woman in the portrait is, as well as what happened to the painting during the Nazi era.

It is believed to depict one of the daughters of either Adolf or Justus Lieser, who were brothers from a wealthy family of Jewish industrialists.

Art historians Tobias Natter and Alfred Weidinger say the painting is of Margarethe Constance Lieser, the daughter of Adolf Lieser.

But the Kinsky auction house in Vienna, which auctioned the artwork, suggests the painting could also depict one of the two daughters of Justus Lieser and his wife Henriette.

Henriette, who was known as Lilly, was a patron of modern art. She was deported by the Nazis and died in the Auschwitz concentration camp during the Holocaust.

Her daughters, Helene and Annie, both survived the Second World War.

The auction house said in a statement that the exact fate of the painting after 1925 was "unclear".

"What is know is that it was acquired by a legal predecessor of the consignor in the 1960s and went to the current owner through three successive inheritances."

The identity of the current Austrian owners has not been made public.

The painting was sold on behalf of these owners and the legal successors of Adolf and Henriette Lieser, based on the Washington Principles - an international agreement to return Nazi-looted art to the descendants of the people the pieces were taken from.

Ernst Ploil from im Kinsky told the BBC: "We have an an agreement, according to the Washington principles, with the whole family".

The im Kinsky catalogue described this agreement as "a fair and just solution".

However Erika Jakubovits, the executive director of the Presidency of the Austrian Jewish Community, said there were still "many unanswered questions".

She has called for the case to be researched by "an independent party".

"Art restitution is a very sensitive issue, all research must be carried out accurately and in detail, and the result must be comprehensible and transparent," Ms Jakubovits said.

"It must be ensured that there is also a state-of-the-art procedure for future private restitutions."

Klimt's art has fetched huge sums at auction in the past.

His Lady with a Fan piece sold for £85.3m at Sotheby's in June 2023, making it the most valuable work of art ever sold at auction in Europe.