John stars in the film alongside Moore, who plays the main super-villain for Taron Egerton’s Eggsy.
Speaking to Jimmy Fallon on US TV last night, Moore recalled: “So I’m like talking to Elton John and acting with Elton John and hanging out with Elton John.
“And then finally, I just couldn’t help myself – after a week or so of filming, I just fangirled out.
“I said, ‘oh my God, oh my God, I just have to tell you that once I went to a concert of yours and I was 16 years old and a friend of mine was a roadie and I was backstage and you walked by me!’
“And then I was like, ‘… The end.’ That was my whole story.
“And he was so nice. He must get it all the time.”
The acclaimed actress added that “he’s so good” in the sequel, which also stars Colin Firth, Channing Tatum and Halle Berry.
Speaking out on her character, Moore teased: “She’s like a drug king pin, she’s created this whole American village around her.
“She’s charming but bad.”
The first Kingsman film – which also starred former EastEnders star Samantha Womack – was released in 2014 and grossed over $414 million worldwide.
Kingsman: The Secret Service is out on September 20 in the UK.