Andrew Lloyd Webber is selling four West End theatres to a consortium led by former ITV chairman Michael Grade.
The composer and impresario said it had been a "gut-wrenching decision" to let the Palace, Her Majesty's, Cambridge and New London theatres go.
"However, following my illness last year, I was advised to reduce the debt in the family company," he said.
The 62-year-old's Really Useful Group will retain the Theatre Royal Drury Lane and the London Palladium.
It will also maintain its 50% interest in the Adelphi, currently home to Lord Lloyd-Webber's Phantom of the Opera sequel Love Never Dies.
The deal is believed to be in the region of £50m, according to reports.
In a statement, Lord Lloyd-Webber said he had reduced the purchase price by £5m to enable the GradeLinnit consortium to invest the same sum in the theatres.
"My commitment to composing, producing and theatre ownership remains as strong as ever," said the composer, whose hit shows include Evita, Cats and Jesus Christ Superstar.
The Really Useful Group (RUG) previously sold its four West End playhouses - the Lyric, Garrick, Apollo and Duchess - to Nimax Theatres in 2005.
Last year, Lord Lloyd-Webber revealed he had been receiving treatment for prostate cancer. He was subsequently re-admitted to hospital after developing an infection.
"Following Andrew's illness last year, both Andrew and his family felt that de-gearing RUG would make it easier for him to concentrate on composing," said the group's chairman Mark Wordsworth.
Theatrical agent Michael Linnit is part of the consortium that, according to Lord Lloyd-Webber, made "an unsolicited approach" to RUG.
The composer said it was "particularly difficult" to part with the New London, which hosted the West End production of Cats for 21 years.
Michael Grade's father Leslie was a theatrical agent, while his uncles - Lew Grade and Bernard Delfont - were both theatre impresarios.
Wordsworth said proceeds from the sale - expected to be finalised in January - would be used to "invest in and develop the Palladium and the Theatre Royal Drury Lane, as well as reducing the Group's debt."
Lord Lloyd Webber is currently working on a new adaptation of The Wizard of Oz, starring Michael Crawford as the Wizard.