Caroline John: Actress best known as Liz Shaw in 'Dr Who'

As Liz Shaw, companion to the third incarnation of Doctor Who, played by Jon Pertwee, Caroline John brought to the programme an intelligence not previously seen in the Time Lord's assistants. On top of tackling action sequences with the usual gusto displayed by them, the scientist could understand the Doctor's techno-babble and he treated her as an equal.

John once recalled how she prepared herself for the role by consulting a dictionary of scientific terminology, only to find the jargon in the scripts to be made up. She also explained that she had no driving licence at the time, despite being seen at the wheel of Bessie, the Doctor's canary-yellow roadster. However, it was the screen scientist's equality with the doctor that might have led to the ending of John's short, six-month run in 1970. She began as Pertwee was taking over as star of the cult series from Patrick Troughton, whose assistants – played by Frazer Hines and Wendy Padbury – had also been written out.

Derrick Sherwin, the programme's script editor, had been promoted to producer the previous year and sought to revamp what the BBC saw as one of its flagging series. He wanted to ditch what he saw as "crazy and unbelievable sci-fi creations which children and hardly [any] adults could even believe in as the basis for a story" and sought to "bring it down to Earth" in the vein of the writer Nigel Kneale's Quatermass serials of the 1950s.

This aim was partly achieved by creating the fictional UNIT, the United Nations Intelligence Taskforce, an organisation defending the Earth from alien threats, of which Dr Elizabeth Shaw was a civilian member, drafted in as a scientific adviser from Cambridge University by Brigadier Alistair Gordon Lethbridge-Stewart (Nicholas Courtney).

When a new producer, Barry Letts, took over a month into John's run and believed that Doctor Who had strayed too far from the original premise of the Time Lord's companions asking him questions that might be in viewers' minds, he decided not to renew her contract. In fact, she was pregnant and would have left anyway.

Nevertheless, John had made an impact and later revived Liz Shaw in the television film The Five Doctors (1983), to mark the series' 20th anniverary, and the 1993 Children in Need special Doctor Who: Dimensions in Time, as well as exclusive video dramas (1994-96) and audio books (2007-2012).

The actress was born in York, the daughter of Anthony John, a theatre director, and Mary, a dancer. She made her film début at 14 as one of a group of food-throwing children at a party in the comedy Raising a Riot (1955), starring Kenneth More. After spending a year in France as an au pair, she trained at Central School of Speech and Drama, before acting at the Royal Court and in repertory in Ipswich, Oxford and Sheffield. She also played Isabella in an RSC production of the Jacobean tragedy Women Beware Women, at the New Arts Theatre Club, London (1962).

John enjoyed four seasons with the National Theatre Company, under Laurence Olivier as artistic director, and when the Northcott Theatre, Exeter, opened in 1967, John starred as Portia in The Merchant of Venice, the first of various leading roles she took in rep after the National. She appeared only twice on TV – as a nurse in The Black Madonna (1963) and in a one-off role in The Power Game (1969) – before landing her part in Doctor Who.

John had a run as Helène Renard, the Resistance leader's wife captured by the Germans, in the third series (1990) of the espionage drama Wish Me Luck and played the housekeeper to the bickering politicians Freddie and Jack in Harry Enfield's Television Programme (1990). She was also seen as the judge in EastEnders (1995) when Nigel Bates fought for custody of his stepdaughter Clare in a battle with her real father.

John and her husband, the actor Geoffrey Beevers, appeared together on stage and screen together many times, such as in the series A Very British Coup (1988) and Agatha Christie: Poirot (1989). Her final television role was in an episode of Doctors four years ago.. She is survived by Beevers and their two sons and daughter, Daisy Ashford, who acts under the surname she took from the author of the novella The Young Visiters.

Caroline Frances John, actress: born York 19 September 1940; married 1970 Geoffrey Beevers (two sons, one daughter); died London 5 June 2012.