American couple Mike Metz and Ashley Millican, both aged 20 and visiting their UK-based relative from Wisconsin, were waiting to see the Queen, Prince Harry and his fiancee attend St Mary Magdalene Church on the Queen's Sandringham Estate for the Christmas Day service when Mike got on one knee.
Asked by Sky News royal correspondent Rhiannon Mills why it was so important for them to come and see the royals on Christmas morning, Mr Metz got on one knee and said: "I want to spend the rest of my Christmases with you Ashley, so I don't know if you want to marry me?"
Putting her hand to her mouth and fighting back tears, she replied: "Yes. Oh my God."
Taking the ring from the little white box and weeping, she said: "Yes, yes, yes."
Mills, who had been tipped off about the proposal, asked how them both how they were feeling, to which Mr Metz said: "I'm feeling great, excited."
Ashley, who said she was not expecting it, replied: "Good, it's my grandmother's ring, it's very pretty."
Presenter Kay Burley, in the studio, said: "Unbelievable, fantastic and thank goodness she said 'yes'."
Mr Metz said the couple planned to tie the knot in around 18 months' time and would "absolutely" watch the royal wedding.
Burley ended the item saying: "That's amazing, and thank goodness she had her nails done as well."
Miss Millican, from Texas, told the Press Association the proposal came as a complete surprise.
"I had no idea," she said, adding: "I was definitely very surprised. I never thought he would ask me right before we were about to see the Royal Family for the first time.
"It was a big surprise."
She said she did not hesitate to say yes.
It is her first holiday outside America and she said they had a two-week trip planned in which they would visit London.
Mr Metz, of Wisconsin, said he started to plan the proposal in August, asked Miss Millican's parents in September and was nervous ahead of the day.
"It was pretty tough to keep secret as I was so excited," he said.
He added: "It's memories to cherish forever."