“Then I go, ‘Stop!’ because I’m singing ‘Eleanor Rigby’! I’ve separated myself not only from Paul and fame, but a couple of bits in my head are going in different places.”

He added: “Sometimes that breaks down and I forget the song. I have a teleprompter.”

McCartney said in an interview last year that he had to “re-learn everything” from The Beatles’ extensive catalogue, with some of the material dating back 55 years.

“We go in rehearsal and I’m like, ‘Oh yeah, that’s how it goes’,” he explained.

“Some of the old songs you say, ‘Oh, that’s clever, I wouldn’t have done that’. It’s exciting to think that still works. We were a little rock and roll group from Liverpool, it just kept going.”

McCartney announced his long-awaited ‘McCartney III’ album last month. Following on from ‘McCartney’ (released in 1970) and ‘McCartney II’ (1980), the new record is set to arrive on December 18.

A press release explained that the musician initially hadn’t planned to release an album in 2020, “but in the isolation of ‘Rockdown’ he soon found himself fleshing out some existing musical sketches and creating even more new ones”.

“Before long an eclectic collection of spontaneous songs would become ‘McCartney III’: a stripped back, self-produced and, quite literally, solo work marking the opening of a new decade, in the tradition of 1970’s ‘McCartney’ and 1980’s ‘McCartney II’.”