The Gavin & Stacey star came across a satchel full of amorous, teenage letters from her grandfather Henry to her grandmother Anita while exploring her family’s history on the BBC genealogy show.
Reading one of them out, which was written in Welsh, Jones said: “Thank you for the photograph, but I must say it isn’t very true to life. You are far prettier in reality. Well baby, oh this life I do so want to be home so very much.
“Your birthday is nearing I believe, please don’t laugh at the enclosure. It is the best Maltese lace I could get. I feel hot all over when I think what uses you will put it to.”
Pausing, Jones laughed and said: “Oh I say, Henry Richard!”
She continued: “This one doesn’t have a date. But it says, ‘My darling sweetheart, to think that only a few days ago we were asleep in one another’s arms. I cannot help thinking of Sunday night.’”
Jones joked: “So erm, things have clearly progressed from first base!”
Jones’s appearance on Who Do You Think You Are? followed episodes featuring Jodie Whittaker and David Walliams.
During Whittaker’s episode, she discovered some uncomfortable truths about her ancestry.