Nicholas Courtney, who played Brigadier Alistair Gordon Lethbridge-Stewart in more than 20 classic Doctor Who stories, has died, aged 81.

His first appearance in Doctor Who was as Space Security Agent Bret Vyon alongside William Hartnell in The Daleks’ Master Plan in 1965, and he returned three years later as British Army Colonel Lethbridge-Stewart in the Patrick Troughton adventure The Web of Fear. Originally cast as Captain Knight in that story, he was promoted when the actor originally cast as Lethbridge-Stewart pulled out.

Courtney was promoted again on his next appearance in 1968’s The Invasion, with Lethbridge-Stewart now a Brigadier serving with the newly formed UNIT (United Nations Intelligence Taskforce), a set-up that would provide the backbone of the series in the early 70s.

With the Third Doctor exiled to Earth, Courtney became a series regular between 1970 and 1974, with the Brigadier in charge of the ‘UNIT family’ that included Sergeant Benton (John Levene), Captain Yates (Richard Franklin), Liz Shaw (Caroline John) and the Doctor (Jon Pertwee) as UNIT’s scientific advisor. One of the most fondly remembered periods of the show’s history, these Earthbound stories also introduced the characters of Jo Grant and Sarah Jane Smith.

When Tom Baker took over as the Doctor in 1974, UNIT and the Brigadier served as a familiar bridging device for his first story, Robot. Producers were keen to return the show to its planet-hopping origins, however, and the UNIT family was largely retired from then on. This was not the end for the Brigadier, though, whose immense popularity led to the character being brought back for three more regular stories: Terror of the Zygons (1975, with Tom Baker), Mawdryn Undead (1983, with Peter Davison) and Battlefield (1989, with Sylvester McCoy), as well as the 20th Anniversary special, The Five Doctors in 1983.

Though Courtney did not appear during Colin Baker’s original run as the Sixth Doctor, this was rectified in the Children in Need special, Dimensions in Time in 1993, and in the Big Finish audio stories, where he also appeared alongside Eighth Doctor, Paul McGann. As well as working with all the classic Doctors, he also performed with David Tennant in two Big Finish dramas, in which Tennant, ironically, played a UNIT Colonel.

In 2008, Courtney made his final appearance as the Brigadier, in the Sarah Jane Adventures story, Enemy of the Bane. Reunited with his old friend Sarah Jane Smith (Elisabeth Sladen), the retired Sir Alistair helped to save the world one last time, introducing the character to a whole new generation of fans.

Born William Nicholas Stone Courtney in Egypt in 1929, Courtney trained at the Webber Douglas School of Singing and Dramatic Art in London in the 1950s, after completing 18 months of National Service as a Private in the British Army. His first TV work was in the 1957 series Escape, and he went on to appear in shows such as The Avengers, The Champions and Randall and Hopkirk (Deceased).

After Doctor Who, Courtney continued to act, in The Bill, Doctors and Casualty, among others. In 1997, he was made Honorary President of the Doctor Who Appreciation Society, and he continued to make regular convention appearances until last year. He is survived by his wife Karen and two children, Philip and Bella.