The items will be sold in one of the charity's 450+ high street shops across the country, to raise money for its Spread the Warmth campaign.
Designer Julien Macdonald is also donating coats, and there will be brand new designs from high street names including Topshop, Barbour, Ted Baker, New Look, Wallis, Burton, BHS, Miss Selfridge, Jaeger, French Connection, Dorothy Perkins, River Island and H&M.
King said: "Donate a Coat is a wonderful cause and I'm really pleased I can lend my support to Age UK. I hope whoever buys my coat loves it as much as I do."
Halliwell added: "Everybody deserves to keep warm this winter. If you buy my coat it will really help Age UK. Big hug, much love, Geri."
Hugh Forde, Managing Director of Retail, Trading and Training at Age UK, said: "It's great that our Donate a Coat appeal has already got such high profile support from celebrities and top fashion brands.
"We hope people across the country get inspired to support us too - by simply popping into one of our shops to donate or buy a coat you really will be helping to make life better for older people in winter."
Visit www.ageuk.org.uk/donateacoat to find out which coats are available and where to purchase them.