A British man has found himself at the forefront of a £100m Holocaust restitution claim for a vast collection of paintings by Gustav Klimt and Egon Schiele.

The 70-year-old doctor became the executor of his late cousin's estate - and discovered that she was the heir to the fortune of Vienna Jewish art collector Jenny Steiner. Her collection of Klimt and Schiele works was confiscated by the Nazis during the Holocaust.

The man, who refused to be named for fear of unwanted attention, said: "It was quite a bombshell. My cousin was in hospital, but she died the day that the news came that she could have a claim to the assets of the Steiner family. I received a phone call from the Federation of Austrian Jewish Communities. It was completely out of the blue, I'd never heard anything about the family before.

"My cousin was married to an Austrian man who had converted to Catholicism, but he was born Jewish. When he died, his mother Daisy wanted her daughter-in-law, my cousin, to inherit her assets under Austrian law, so she adopted my cousin.

"Adopting your daughter-in-law is very strange, but it's lucky because that's how our family became the heirs to the Steiner assets. My cousin's husband was the grandson of Jenny Steiner."

As executor of his cousin's estate he now handles all the restitution claims for the family. He has more than 25 relatives all over the world who have a claim to the paintings. The first to be recovered was Klimt's Landhaus am Attersee in 2001, sold for $29m by Sotheby's in Geneva. Three paintings by Schiele, worth $6m each, have also been restituted, but he believes there are about 14 more to be found.

He said: "I'm not exactly a connoisseur, I never knew a thing about art before. I had to learn very quickly about art and about family history. It's been an extraordinary experience, and I have no idea when it will end. But it's kept me busy in my retirement, for sure. I'm the sort of hub for communication about everything!

"I couldn't guess at how much the full estate is actually worth, and obviously it has to be divided among many relatives. But the Klimts go for more than £20 million each, so we will see what happens when the other claims go through."

Other paintings known to have been in the collection include Schiele's Haus am Meer, which was on show in the Leopold Gallery, Vienna, and Mother and Two Children.

Jenny Steiner fled to New York but her sister Irinka, who also collected Klimt's paintings, was killed in the Holocaust. A third sister, Serena, escaped from Austria but died before the end of the war. Klimt, in fact, painted a portrait of Serena. All three sisters were married and there are heirs to each branch of the family.

Mike Fleming, of chartered accountants Straughans Ltd in County Durham, has been helping to handle the estate. He said: "It's scandalous really how difficult it has been to deal with Austrian law, and with the galleries which currently hold the paintings. The people who run these galleries know full well where these paintings came from, but they resist giving them back."

Recently Mr Fleming won a landmark claim for the family against the Inland Revenue, which said that the executor owed 40 per cent inheritance tax on his cousin's assets - and he could be charged for however much the authorities believed her Austrian inheritance was worth.

He said: "As far as we are aware this is the first time we have reached agreement with the British tax authorities about restitution claims. The family will now pay no tax on any new paintings that are discovered. It sets a great precedent for any other families in the UK struggling with restitution claims and taxation."