The Saturdays singer was overwhelmed when she and boyfriend Ben Foden were introduced to the royal couple at the recent wedding of William's cousin Zara Phillips and Mike Tindall, who plays rugby with Ben.
She said: "It was a huge honour. I got an invite because Ben is friends with them. I sat about 10 metres away from the queen.
"I was introduced to Kate and William and was literally speechless. I was a little bit starstruck."
Una insists she has no desire to settle down and get married, despite attending several friends' weddings this year.
She told LOOK magazine: I've been to three weddings this year, including Zara's. She went out with Mike for nine years before they got married.
"Ben and I have been together for two and a half years - I don't see any rush for it."