David Gollancz found out in 1965 at the age of 12 that the father he had always known was not, in the biological sense, his father.

"My father said he had something important to tell me and we went and sat down in my bedroom and he explained that he and my mother had found that he was infertile and they had gone to a doctor who had provided sperm from another man - and that was how I was conceived," he said.

It came out of the blue. "It felt like being hit by a train. I just felt this massive impact like a hammer blow."

He describes then feeling "a huge mental silence" because his father could not answer the obvious question. To "who is my Dad, then?", he could only respond, "I don't know and you can't find out."

The only information Mr Gollancz's parents had was that the donor was Jewish, like his father.

They had gone for help to Mary Barton, a pioneering gynaecologist who was one of the first doctors in the country to assist with fertility treatment - and her condition was that the donor remained anonymous.

Mr Gollancz, a lawyer, did not at first feel any burning need to know his genetic father's identity, but now "I want to know much more than I did then", he said. "As I get older, it becomes much more important to me."

This is partly because he has met, through an amazing chain of coincidences, two half-siblings.

The brother and sister, who live in North America, share a mother who conceived thanks to donor sperm from Dr Barton's clinic. Mr Gollancz met them and liked them and one day they had a DNA test done. It showed a 99% likelihood that they were genetically related.

He has complex feelings about donor insemination, but believes the loss of anonymity will make all concerned think harder before they go ahead. And he believes the children who result have a right that has until now been overlooked.

"I think people have a right not to be deliberately deceived about, or deliberately deprived of, essential elements in their personal history," he said.