Diane Keaton died from pneumonia.

The actress passed away on October 11 at the age of 79 and her family have now revealed her cause of death in a new statement, in which they thanked fans for their “extraordinary” messages of support.

Keaton’s family told People magazine: “The Keaton family are very grateful for the extraordinary messages of love and support they have received these past few days on behalf of their beloved Diane, who passed away from pneumonia on October 11.”

The family suggested those who wanted to pay tribute to Keaton could make a donation to one of her favourite causes.

They added: “She loved her animals and she was steadfast in her support of the unhoused community, so any donations in her memory to a local food bank or an animal shelter would be a wonderful and much appreciated tribute to her.”

It was previously reported that Keaton’s health had “declined very suddenly” before her death.

A source told People magazine: “She declined very suddenly, which was heartbreaking for everyone who loved her.

“It was so unexpected, especially for someone with such strength and spirit.

“In her final months, she was surrounded only by her closest family, who chose to keep things very private. Even longtime friends weren’t fully aware of what was happening.”

Diane Keaton has died aged 79 after a sudden battle with pneumonia. Photo / Getty Images

In March, Diane put her “dream home” in Brentwood, California, up for sale, despite previously vowing to stay there forever, and she was no longer regularly seen in the area walking her dogs.

Another source said: “She lived in Brentwood for many years.

“She loved her neighbourhood. Up until just a few months ago, she’d walk her dog every day. She was usually dressed the same, with a hat and her signature sunglasses regardless of the weather.

“She was always very nice, funny and chatty. She’d talk to her dog like he was a person. She was eccentric and had this old-school Hollywood aura. She was very, very special.”

A pal remembered Keaton as “one in million” who was “funny right up until the end”.

The friend said: “She lived exactly how she wanted to, which was on her own terms, surrounded by the people and things she really loved ...

“Diane always had a very strong sense of who she was and how she wanted to live.

“In these last few years, she kept a close circle and she liked it that way. She was funny right up until the end and she had this way of making even ordinary moments feel special. That was just who she was.”

Keaton never married but is survived by her adopted children, daughter Dexter, 29, and son Duke, 25.