The instrument "powered Beatlemania and shaped the sound of the modern world", according to those behind the Lost Bass project.

The Hofner 500/1 electric bass was purchased by Beatles star McCartney from a shop in Hamburg in 1961 and it was reportedly his "favourite".

The musician played it at the Top Ten Club in the German city that same year, at Liverpool's Cavern Club, and on the Beatles' first recordings at the famous Abbey Road studios.

He played it on the Beatles hits Love Me Do, She Loves You, and Twist And Shout.

McCartney then put it to one side for several years, but in early 1969 he picked it up again while the band were in London recording the Get Back/Let It Be sessions.

The instrument could also be seen in Get Back, the Peter Jackson documentary which was released in 2021.

It was around the time of the sessions in January 1969 that the guitar went missing, a year before the band split up.

Speaking in a 1966 Beat Instrumental interview, McCartney said: "I have had a Hofner ever since I started. I've got three models but the ancient one is still my favourite. It has seen so much work that some of it is held with sellotape!"

There are various rumours about what happened to it, including that a thief took the bass from a closet at Abbey Road, or that it went missing from the basement of the Beatles' Savile Row offices.

Read more:
Mother of Beatles star 'ashamed to be female'
AI used to create 'last Beatles record'

Those behind the new project say they are searching for "the most important bass in history" and they call its disappearance "the greatest mystery in rock and roll".

According to Hofner, McCartney kept his first violin bass until it went missing, using it as a back-up during the Beatles' world tours and playing it again in the Revolution promo video and during the filming of Let It Be.

'No other 500/1 bass looks like this'

The company said on its website: "The bass was in need of repairs and so it was sent to a firm in London, early in 1964, who carried these out. They resprayed the bass a darker three-part sunburst and fitted new knobs.

"Most significantly they custom-made an unusual single pickup frame to hold both pickups. It was very distinctive, no other 500/1 bass looks like this."

'Paul would be so happy if this bass could get back to him'

One of the people involved in the hunt is Nick Wass, a bass expert and Hofner executive, who said the guitar "was played in Hamburg, at The Cavern Club, at Abbey Road. Isn't that enough alone to get this bass back?"

"I know, because I talked with him about it, that Paul would be so happy - thrilled - if this bass could get back to him".

The most expensive guitar ever sold was singer Kurt Cobain's Martin D-18B which was bought at auction in Los Angeles in 2020 for $6m (£4.77m).

It was used during Nirvana's famous MTV Unplugged set - a gig which was televised and universally lauded.