Sladen had been suffering from cancer, BBC entertainment reporter Lizo Mzimba said. It had not been widely known that the actress was in ill health.
He said: “She was one of the most popular Doctor Who companions of all time, and one of the people who managed to do both old Doctor Who and Doctor Who when it came back.”
She has been active in the Doctor Who series since 1973 staring opposite the 3rd Doctor Jon Pertwee, Stayed on two years with Tom Baker, appeared with the 5 doctors special with 5th Doctor Peter Davidson, Appeared in audio adventures for Big Finish (including Zagreus with the 8th Doctor) and in the Dimensions in Time Special along with Doctors 3-7, Came back triumphantly in the Second Series of the New doctor Who series appearing 4 times with David Tennant, and during the run of her current series the Sarah Jane Adventures appeared opposite both Nicholas Courtney (the Brigadier) Katy Manning (Jo Grant) and 11th Doctor Matt Smith. If she had lived there is no doubt the series would have continued as long as she did.
Update: At Big Finish Nick Briggs reveals that she was going to be doing new audio adventures with Tom Baker at Big Finish if she had lived, how wonderful those would have been.