Ian Smith leaves tv show Neighbours after nearly four decades as ...

Australian actor Ian Smith has announced he has left his longtime role on television series Neighbours following a terminal cancer diagnosis.

The 87-year-old has played Harold Bishop on Neighbours since 1987, along with a dozens of roles across Australian television series over nearly six decades.

In an interview with 10 News First, Smith said he was notified several months ago he was suffering from a "very aggressive, non-fixable" lung cancer.

He revealed he had since left the role to focus on his health.

Smith said he had been through three rounds of treatment, including immunotherapy and chemotherapy.

"They expect me to die," he told 10 News First.

"I've really put my hand up just to be a guinea pig, I think, plus the fact that I don't want to die."

Smith's biological mother, Peggy Kline, and his wife Gail Smith also died from cancer.

Ian Smith has left Neighbours following his cancer diagnosis. (Supplied: Network Ten)

A Channel Ten spokesperson said the network was deeply saddened by Smith's diagnosis.

"A cherished member of the Neighbours family, Ian has been the heart and soul of the show since he first arrived on Ramsay Street in 1987," the spokesperson said.

"Ian has brought joy, laughter and inspiration to audiences all around the world for decades.

"Along with the cast and crew and all his fans from around the world, we will support Ian in any way we can during this difficult time."

Jason Herbison, who has served as executive producer of Neighbours since 2013, said the show would give the character of Harold a proper send-off in coming episodes.

"The way the residents of Ramsay Street feel about Harold is how we all feel about Ian — we couldn't love him more than we do," Herbison said.

"Despite the challenges he (Smith) was facing with his health, he wanted to give Harold a proper send-off. It was a very emotional moment on set as we paid tribute to him and the incredible contribution he has made to the show."