Taylor Swift has revealed it only took around eight hours to write and record her Toy Story 5 song I Knew It, I Knew You.

The Fate of Ophelia hitmaker has gushed that she had "one of the most fun days of my life" making the end credits tune for the latest instalment in the beloved Pixar franchise and revealed it was all wrapped up within hours of her seeing the film, which is in cinemas now.

Sharing an insight into the whirlwind process, she shared in a video from the studio posted to her Instagram: "Been kind of a hectic day.

"At 11am, went to go see Toy Story 5, got so inspired, got the songwriter zoomies, went home, wrote the end credit song for Toy Story 5.

"We have now produced it, and I'm doing vocals. It's 6:57pm. In two hours, [Disney's] Bob Iger and Tom from Pixar are coming to hear it.

"We have not recorded it yet. And I think this is one of the most fun days of my life."

Taylor's involvement with Toy Story 5 was kept "top secret" and even Tom Hanks - who leads the voice cast as cowboy Woody - was in the dark about it.

He told Variety: "It was top secret. We did not know until, literally, when the moment came. They ushered us into a soundproof room and said, 'Tonight at 9pm, the true end titles song is going to drop, and it's by Taylor Swift.'

"And I was like, 'You guys kept this from us all?' We saw the movie without [the song], we had some dummy thing in there. They surprised us as well, to a delight.

"That's like saying, 'By the way, Judy Garland is singing 'Over the Rainbow' at the beginning of this.' That kind of thing."

I Knew It, I Knew You saw Taylor return to her country roots, and the lyrics are written from the perspective of cowgirl Jessie, who she adored and dressed up as a kid.

When the song was announced, the 36-year-old pop idol admitted on social media she had "always dreamed" of working on the Toy Story soundtrack.

She wrote alongside a video of herself dressed as Jessie as a child: "I've always dreamed of getting to write for these characters who I've adored since I was a five year old kid watching the first Toy Story movie.

"I fell instantly in love with Toy Story 5 when I was lucky enough to see it in its early stages, and I wrote this song as soon as I got home from the screening. Sometimes you just know, right?

"Writing this song felt like a musical departure and coming home at the same time.

"Creating something for Jessie was a new challenge and also felt like second nature all at once. And being a @toystory kid from the age of five til now… is an adventure I plan to be on, to infinity and beyond."