Daniel Radcliffe is choked to ­discover that his great-great-uncle Ernie – whose photo inspired him during filming of First World War drama My Boy Jack – was killed during the conflict.

The Harry Potter star learns during filming of BBC1’s Who Do You Think You Are? that Ernie McDowell was one of great-great-grandmother Flo’s four brothers who went off to fight.

Sadly Ernie, from Banbridge in Co Down, Northern Ireland, was the only one not to return home.

In 2007 Radcliffe played John Kipling in an ITV film about the grief of his author father, Rudyard Kipling, when the 18-year-old died in the conflict.

On the show, he explains: “I kept a picture of Ernie in my trailer as a personal connection to the period.”

He also discovers that letters between Ernie, his mother Elizabeth and his sweetheart Jeanie have been kept and passed down in the family.

And viewers will see him almost moved to tears to read Elizabeth urging her son to come back safely “to the same loving mother that you left behind”.

In another love letter from his sweetheart Jeanie, she tells him: “I love you Ernie with all of my heart – with a love that will never die”.

Radcliffe, 29, whose father Alan also grew up in Banbridge, is later thrilled to discover that Ernie and Jeanie managed to marry in 1915, when he was recuperating in Ireland from frostbite and a leg wound.

But the couple’s ­happiness was not to last. Following his return to the front, the letters dried up in 1916.

The actor finds that a soldier from Ernie’s unit had written to Elizabeth, explaining what had happened.

“We were just after arriving in the trenches and your boy and two more chaps from Belfast was going into a dug out to take off their packs when a shell landed which killed the three of them. I’m very sorry to say none of them did live to say a word to anyone,” he tells her.

Daniel reasons that the young man’s words would have brought some comfort, saying: “That’s extraordinary. He wants to reassure Elizabeth that Ernie didn’t suffer.”

He concludes: “To have played at being a soldier in the trenches, I definitely feel a lot more connected to all those stories now I’ve learned what my own family went through.

“It’s given me such an insight into what it would have been like to have your children leave for the war.”