Dame Judi will sing Send In The Clowns from A Little Night Music at the concert, which celebrates Sondheim's 80th birthday year.

On hearing her performance of the song during the National Theatre's 1995 revival of the show, Sondheim is said to have told the acclaimed actress: "It's yours now."

Dame Judi said: "Singing at the Proms is not something I ever imagined I would do and it's a hugely exciting - if slightly daunting - prospect."

She hailed the composer as a "genius" and continued: "His impact on musical theatre can never be overstated and I'm delighted to be part of his 80th birthday celebrations at the BBC Proms."

The glittering line-up for the Sondheim celebration includes Proms favourite Bryn Terfel alongside Simon Russell Beale, Daniel Evans, Maria Friedman, Caroline O'Connor, Julian Ovenden and Jenna Russell.

Numbers will also be performed from Follies, Sunday In The Park With George, Into The Woods, The Frogs, Company, A Funny Thing Happened On The Way To The Forum and Sweeney Todd.

Sondheim will be present at this Saturday's concert, which will be broadcast live on BBC Radio 3 at 7.30pm and later on BBC2 at 8pm.

The Sondheim at 80 Prom marks another first in Proms history.

It will be the first Prom for deaf concert-goers and is signed by Paul Whittaker, who is profoundly deaf and has worked on West End productions such as Les Miserables and Grease.