Gavin and Stacey star Alison Steadman was left shocked and upset during filming for ITV's DNA Journey, as an emotional discovery changed everything for her family.
The actress joins pal and co-star Larry Lamb on the series, which delves into celebrities' ancestry and their DNA.
But Alison revealed on Good Morning Britain on Tuesday that she discovered something on the series that had never been revealed to her family, that left her "upset" and "knocked sideways".
Alison found out that her late father was actually adopted, and his parents were not his biological family.
She believes her dad never knew the truth, and no one in the family did until the revelation came out during filming - apart from Alison's grandmother, aunt and uncle.
Alison told hosts Ed Balls and Susanna Reid: "My father was adopted, which I didn't know, none of us knew in our family."
Larry added: "It knocked her sideways!" as Alison went on: "I don’t think [my father] knew.
"He was eight months old. I was so glad that I had Larry with me honestly because I was just so upset by it, I really was.
“My grandmother, my aunt, my uncle they all knew [and] they didn't tell me, but they were lovely people that's the main thing and I love them to bits. But to find out suddenly that my grandma wasn’t my grandma as it was, was quite upsetting."
Larry shared that he had a similar family discovery in his past, revealing: "I’d found out the same sort of thing - my mother was adopted.
"And there was a whole chain of things that sprang off that, so I kind of got where Alison was. Mine was kind of easier to accept but Ally’s, this was a big shaking of the keystone."
Alison and Larry also discussed hopes for a Gavin and Stacey return, with them suggesting it wouldn't happen for a while if at all.
Larry said of the rumours of a Christmas special being in the works: "I wouldn't hold your breath - and certainly not this Christmas!"
The pair also revealed they had appeared in another show together before they landed the roles of Pam and Mick in Gavin and Stacey.
Alison said: "We’d done a telly thing together but we didn't have any scenes [together] in it."
Larry then asked Alison: "Did I play your husband in that?! But I think I only had one line in it or something," to which Alison replied: "I can’t remember!"
Good Morning Britain airs weekdays at 6am on ITV and ITV Hub.
