In a statement published on the BBC website, Moffat said: "Caroline was a brilliant actress and in her role as Liz Shaw, a tremendous co-star for Jon Pertwee in his first year as the Doctor. She was not just a sidekick but a scientist in her own right and a match for the universe's number one know-all. The Doctor's companions should never be his assistants - they're the people who keep him on his toes, and that's what Caroline did. From everyone at the show, our thoughts go out to Caroline’s family."
Sixth Doctor Colin Baker wrote on Twitter: "Devastated to hear that the lovely, talented, wise and gentle Caroline John is no longer with us. So sad. Thoughts with Geoffrey and family."
John's Doctor Who successor, Katy Manning, said: "just heard about Caroline John ! She was a dear beautiful soul .her humour &talent will be sorely missed xxxxxxx"
Nicola Bryant added: "Very sad news. A lovely one has left us. All I can say. :("
Frazer Hines expressed his sadness at the passing of the actress: "so so sad to hear the news. Another of our family gone."
Mark Gatiss, who worked with Caroline John on the P.R.O.B.E series, tweeted: "Very sad to hear that the lovely Caroline John has passed away. RIP."
Barnaby Edwards said: "Very sad to hear about the death of Caroline John. Such a brilliant, intelligent actress."
Anneke Wills tweeted: "Such sad news about Carrie John. She will be sorely missed. Love to Geoffrey and the family. xxxx"
Toby Hadoke has written an obituary for Caroline John which has been published on The Guardian website.