The ITV1 show's creator and writer Derren Litten confirmed the news while speaking at Attitude magazine's Sweet 16 party, describing the pair's involvement as "really exciting".
Litten told The Mirror: "Both women are great signings for the show and I can't wait to work with them. Cilla has basically agreed to play the oldest swinger in town and will really ham it up in a basque and suspenders, all the works.
"The show will be going out in autumn and will be very special. I'm really proud of it."
He added: "I've always wanted to work with Katie too, so we've been in touch with her people and offered her a role playing a completely chavvy Brit abroad, getting them out for the lads and lying by the pool. She probably won't have to do too much acting. It will be more sunbathing and bikini action."
Benidorm's autumn outing will be a one-off special, while a fourth series will follow in 2011.