John Lennon Allegedly Had Affair with Teenage Assistant Set Up by Yoko Ono

John Lennon allegedly had an affair with an assistant who was 10 years his junior and it was all Yoko Ono's idea ... according to a new documentary.

According to "The Lost Weekend: A Love Story," a doc that just premiered at the Tribeca Film Festival ... John had an affair in the 1970s with his assistant, May Pang.

The film, narrated by May and from her point of view, claims May initially met John and his wife Yoko when she was 19 years old and starting a job at The Beatles' music company in New York, Apple Records.

The story goes ... John and Yoko liked May so much they asked her to become their personal assistant, working for them for a few years until one morning in 1973 when Yoko allegedly insisted May go out with John.

The doc claims Yoko told May she was not getting along with her husband and feared John would start seeing other people ... with Yoko suggesting May get involved.

May was 22 and John was 32 when the alleged affair began ... and she claims John had previously told Yoko he found her sexually attractive, though she didn't have romantic feelings toward him when he first pursued her.

The alleged affair lasted 18 months, beginning with John kissing May in an elevator ... and saw John and May move across the country to Los Angeles, where Lennon started making music again with Paul McCartney.

Things even got to the point where John and May were sending joint Christmas cards ... according to the doc ... though they are said to have kept their alleged affair secret from everyone outside their tight friend circle.