Vicky McClure teared up as she discussed he great-grandfather's tragic past while learning he was a Japanese prisoner of war in Taiwan, during World War II.
The Line of Duty star, 41, will retrace her relative's steps on BBC's genealogy show Who Do You Think You Are? airing this evening.
In a clip shared ahead of the programme, the actress welled up as she contemplated her ancestor's Harry Millership's 'horrific' experience.
Reflecting on her job in the spotlight, Vicky explained that she has 'a very vivid imagination', adding that she's 'battling with it' while exploring Harry's struggle 6,000 miles from home.
'My head's all over the shop and I'm learning information that is just horrific, and it's my great grandad, it's my mum's grandad - it's too close,' she said.
Actor Vicky McClure, 41, journeys 6,000 miles to Taiwan to learn more about her great-grandfather Harry Millership's experience as a Japanese POW
Vicky McClure shared a photo of her late great grandfather Harry Millership (second right) for the first time during an emotional episode of BBC's Who Do You Think You Are?
The Line Of Duty actress, 40, is reduced to tears when learning how Harry was captured alongside 130K other soldiers, according to the Mirror
Vicky looked visibly strained as she talked to to the camera about her gut-wrenching family history.
During the episode, she discovered the awful reality of the journey from Singapore - where Harry's regiment had been posted in 1941 - to the Taiwanese POW camp where he was interned after the Japanese attacked on December 8.
Vicky learned that Harry would have been taken in an airless hold with very little in the way of supplies and, upon arrival at the POW camp, forced to work in a copper mine.
To commemorate the 80th anniversary of the D-Day landings earlier this year, the BAFTA Award-winning actor spoke about another family member who served in WWII for an ITV documentary.
Vicky's grandfather Ralph McLure, 97, explained how he narrowly escaped death when his landing craft was hit by enemy fire during the invasion of Normandy.
He was just 17 when he signed up to serve in the Royal Navy and was deployed as a signaller on a landing raft tank on that historic day.
Sharing the respect she felt for her grandfather and reflecting on the importance of remembering D-Day, Vicky said: 'It is so important to keep this history really close and make sure the kids know about it.
'It's so important that people like my grandad are able to recall it and I'm sure there are many veterans who felt unable or didn't want to do that.
'So any kind of information we can have that comes in about that history is important.'
In the action-packed episode Vicky also learned her grandmother Jean (pictured) was given up for adoption after being conceived out of wedlock
Vicky, pictured with her WWII veteran grandfather Ralph McClure, was vocal in an ITV documentary about the importance of remembering what happened on D-Day
Sobbing and declaring he would have been 'absolutely petrified' while being transported via a 'hell ship' with many fellow men dying on route.
In the old black and white image that Vicky shared with fans, Harry is seen wearing his military uniform alongside fellow soldiers.
Vicky cried: 'It's so much to take in but I feel terrible because he was alone and he didn't make it home. I just weirdly feel connected, which is bonkers because I never met him.'
Harry sadly died at the camp after falling 30ft to his death, with the actress taking comfort knowing he died instantly.
Vicky admitted she was nothing like her 'hard as nails' TV characters while tracing her ancestors and travelling to a prisoner of war camp in Taiwan.
She said: 'There's been a lot of it I've not found easy. For all the action-packed shows that I do and the fact that I might seem hard as nails in certain things, I probably couldn't be further from that'.
In the action-packed episode Vicky also learned her grandmother Jean was given up for adoption after being conceived out of wedlock.
Vicky cried: 'It's so much to take in but I feel terrible because he was alone and he didn't make it home. I just weirdly feel connected, which is bonkers because I never met him'
Her adoptive parents terrible abused her, beat her and tried to force her into sex work.
During the show Vicky said: 'It breaks my heart knowing that she wasn't given away to good people. I never knew anyone as happy as her but she had such a bad start. In my head it's her trying to break that chain.'
Harry sadly died at the camp after falling 30ft to his death, with the actress taking comfort knowing he died instantly.
Vicky admitted she was nothing like her 'hard as nails' TV characters while tracing her ancestors and travelling to a prisoner of war camp in Taiwan.
She said: 'There's been a lot of it I've not found easy. For all the action-packed shows that I do and the fact that I might seem hard as nails in certain things, I probably couldn't be further from that'.
In the action-packed episode Vicky also learned her grandmother Jean was given up for adoption after being conceived out of wedlock.
Her adoptive parents terrible abused her, beat her and tried to force her into sex work.
During the show Vicky said: 'It breaks my heart knowing that she wasn't given away to good people. I never knew anyone as happy as her but she had such a bad start. In my head it's her trying to break that chain.'
It comes after Adrian Dunbar teased the return of Line of Duty during an appearance on Good Morning Britain.
Harry was interned in a Taiwanese POW camp after the Japanese attached his regiment in Singapore on 8 December 1941
In the old black and white image that Vicky shared with fans, Harry is seen wearing his military uniform alongside fellow soldiers.
Vicky cried: 'It's so much to take in but I feel terrible because he was alone and he didn't make it home. I just weirdly feel connected, which is bonkers because I never met him.'
Harry sadly died at the camp after falling 30ft to his death, with the actress taking comfort knowing he died instantly.
Vicky admitted she was nothing like her 'hard as nails' TV characters while tracing her ancestors and travelling to a prisoner of war camp in Taiwan.
She said: 'There's been a lot of it I've not found easy. For all the action-packed shows that I do and the fact that I might seem hard as nails in certain things, I probably couldn't be further from that'.
In the action-packed episode Vicky also learned her grandmother Jean was given up for adoption after being conceived out of wedlock.
Her adoptive parents terrible abused her, beat her and tried to force her into sex work.
During the show Vicky said: 'It breaks my heart knowing that she wasn't given away to good people. I never knew anyone as happy as her but she had such a bad start. In my head it's her trying to break that chain.'
Who Do You Think You Are? airs every Thursday at 9pm on BBC One and BBC iPlayer.
