Prince Charles' party, with musicians, comedians and environmental experts, will mark the official public opening of his gardens at Clarence House.
A Garden Party To Make A Difference, is billed as a "rare opportunity" to visit the gardens and will last 12 days from 8 to the 19 September.
Neighbouring gardens at Lancaster House and Marlborough House will also be thrown open to the masses as part of the prince's Start initiative launched in February.
The event aims to show people "the small steps that can, and are, being taken by all of us, interested in building a more sustainable future", said the a rep for the Prince's Charities.
Prince Charles said: "This festival is just one way in which Start can help to demonstrate how we can all make a difference, however big or small the steps we take."
The 12-day garden party will include exhibitions, interactive displays and live performances from comedians, musicians and speakers.
"Celebrity champion curators" include pianist Jools Holland and Westwood, whose iconic images included the Queen with a nose piercing and whose boyfriend, Malcolm McLaren, managed Sex Pistols' songs God Save the Queen and Anarchy in the UK.
One of the sponsors, IBM, is to host a nine-day IBM summit for business and industry leaders in 16 September to look at how companies can tackle environmental sustainability.
Tickets to the garden cost £15 for adults, visit www.startuk.org