Image caption Nicholas Courtney appeared opposite five different Doctors, including Sylvester McCoy (l)

Veteran Doctor Who actor Nicholas Courtney, best known for playing Brigadier Alastair Lethbridge-Stewart, has died in London at the age of 81.

The series regular passed away after a short illness, his agent told the BBC. He had been suffering from cancer.

The actor appeared on screen opposite many different Doctors and worked with some more on Doctor Who audio stories.

Former Doctor Tom Baker remembered him as "a wonderful companion" with "a marvellous resonant voice".

"Of all the characters in Doctor Who there is no doubt that he was the most loved by the fans," he wrote on his official website.

'Nick' Courtney had already appeared in such series as Escape and The Saint before making his first Doctor Who appearance in 1965, playing a character named Bret Vyon.

He returned to the show in 1968 to make his first appearance as the Brigadier opposite the second Doctor, Patrick Troughton.

The stalwart of the United Nations Intelligence Taskforce (UNIT) would periodically return over the next two decades before his final appearance in 1989.

In 2008 he reprised his Brigadier role in Doctor Who spin-off The Sarah Jane Adventures.

Image caption In 2008 he appeared in an episode of The Sarah Jane Adventures

Courtney appeared on other shows during his Doctor Who years, including The Avengers, Callan, The Champions and Minder.

Afterwards he had parts in such popular dramas as The Bill, Casualty and Doctors and was recently heard on BBC Radio 7 series The Scarifyers.

In 1997 he became honorary president of the Doctor Who Appreciation Society in recognition of the 107 episodes in which he appeared.

League of Gentlemen star and Doctor Who writer Mark Gatiss remembered the actor as "a childhood hero and the sweetest of gentlemen".

Impressionist Jon Culsaw said Courtney was "a brilliant actor and warm, charming man", while Shaun of the Dead director Edgar Wright paid tribute to "a true gent".