A former British prisoner of war in Auschwitz has described how he received a cherished oil painting of his sweetheart, painted from a black and white photograph by a Jewish prisoner in the camp.

Welsh-born Bryn Roberts, now 92, who lives in Lyme Regis, was a labourer in Auschwitz when he received a picture postcard of his girlfriend Peggy Gullup through the Red Cross.

A Polish POW saw the picture and offered to have it painted, as Jewish prisoners worked in the camp making copies of stolen artworks. The picture came back two weeks later, a colour oil painting, in the correct shade of Peggy's actual jumper, even though the postcard was in black and white.

Mr Roberts' cousin Peter Lewis submitted the story of the painting to the Radio 4 series A History of the World in 100 Objects, which looked at artefacts in the British Museum. Listeners were invited to suggest their own objects to map world history. The story thrilled producers, who made a one-off documentary about Mr Roberts' painting, broadcast this week on Radio 4.

Mr Lewis, 62, whose first wife and two sons are Jewish, said: "It is an amazing story, which we only really heard in full around ten years ago. Obviously, Bryn didn't believe the Polish POW when he said he could get the photo done in oils, but let him have it reluctantly, warning him that if he lost it then he would break his neck. A fortnight later, the painting came back. There was no payment.

He said it was in the interests of the Jewish painters to keep busy, because otherwise the Nazi guards would question why they were kept alive. The picture was not signed, because obviously the person who did it could be punished."

Although the painter's identity remains a mystery, the family still hope it could be possible that someone recognises the painting. Mr Lewis said: "Once there was a survivors' group of former painters in Auschwitz. I know Bryn wishes he had got in touch with them."

Mr Roberts married Peggy in December 1945, but is now widowed. He told the BBC: "I would have loved to have found out who the painter was. But I have no way of knowing."

He witnessed what was happening in the Jewish area of the camp. "We knew what was going to happen to them. It was all out in the open. I saw the trucks arriving with all the naked people going to their deaths; kids, babies, and women. The Germans were always talking about it."

Mr Roberts was first captured in Belgium in 1940 and spent time in many different camps, including Lamsdorf and Terezin. He kept the painting taped to his stomach and hidden for two years, and he himself remained in captivity until 1945.

He attempted to escape 12 times and was finally successful, a month before the war ended. The painting still hangs in his living room. Mr Roberts said: "It's a damned good painting. I made sure the Germans never got their hands on it."

A History of the World Special can be heard on bbc.co.uk/radio4