Paul McCartney spotted filming 'Carpool Karaoke' in Liverpool with James Corden

June 9 was shaping up to be just another Liverpool Beatles tour workday for tour guide Jackie Spencer, who takes fans to sites in the Fab Four's hometown. But when the small group stopped at the statue of the four Beatles on the Liverpool waterfront, two very unexpected guests joined them, she said.

"We'd had a lovely morning. I had four people with me, from Washington State and California, and we were finishing the tour by the new statues," Spencer told Variety on Sunday.

"There was a couple of paparazzi around and we said to one of them, 'Who's the celebrity in town?' We'd heard rumours but nobody would tell us. And then this guy said, 'Well, I'll tell you one of them: It's James Corden. He's with somebody else and they're in that car over there.'"

She said she believed it was his first visit to the statues, which were sculpted by Andrew Edwards and unveiled in 2015 for the 50th anniversary of the Beatles' final concert in the city.

"He was being really nice to everybody. He was backing people in so they could get pictures with him. He and James Corden were taking selfies. And they just walked back, got in the car and disappeared off and went 'round Liverpool. It was fabulous."

Besides the visit to the statues, he and Corden dropped by Penny Lane, where he put his autograph on one of the street signs and visited his birth home at 20 Forthlin Road. Spencer said someone at the house told her that he went into the house and played 'When I'm 64' on the piano.

While in town, McCartney also did a short show at a pub called Philharmonic Hall and greeted waiting fans with gusto. 

A popular segment on The Late Late Show with James Corden, 'Carpool Karaoke' sees musical guests accompany James Corden on a drive, while singing along (on in some cases lip syncing) to their songs with him.

Previous 'Carpool Karaoke' guests include Britney Spears, Harry Styles, Madonna, the Red Hot Chili Peppers, Justin Bieber and Adele.