The money will fund the purchase and restoration of Holt Hall, near Gresham's School, where Sir James boarded from the age of nine.

Set in 85 acres, Holt Hall will house state-of-the-art facilities for pupils aged seven to 13.

A new prep school is due to open in September 2026.

Sir James said he was "forever grateful" for the generosity Gresham's showed when his father Alec Dyson, who taught classics there, died at the age of 43.

He and his brother were provided with a bursary, enabling them to continue their studies.

Sir James said he was "pleased to be able to support the school and to see it shaping incredible young people who go on and flourish".

He has now donated more than £50m to the school. The Dyson STEAM building opened at Gresham's last year, following a previous donation of £18.75m.

STEAM stands for science, technology, engineering, arts and maths.

There will be a new wing at Holt Hall with a "mirrored external surface" that will "reflect the ancient woodland".

"We will also restore the Victorian building and protect the surrounding ancient woodland for Gresham's pupils to discover and learn in, adding state-of-the-art facilities," Sir James said.

The headmaster at Gresham's, Douglas Robb, said the donation represented a "once-in-a-generation opportunity to rescue Holt Hall and breathe new life into its grounds".

He added: "No land will be sold off during this development and we will repurpose the prep school's existing site to expand the senior school facilities.

"I am particularly grateful that Sir James has chosen to further enhance the school's facilities and build on the tremendous success of the Dyson STEAM Building."