Neil Innes, the English writer, comedian and musician - dubbed "The Seventh Python" - has died aged 75.
He was best known for his work with the Bonzo Dog Doo-Dah Band, The Rutles and collaborating with Monty Python.
A spokesperson for the Innes family said he had not been suffering from any illness and had passed away unexpectedly on Sunday night.
Python founding member Sir Michael Palin described him as "a great writer" and "the most lovely friend".
"He was the first Neil Innes really!" Sir Michael told Radio 4's PM programme. "We were very lucky to have him on board."
"He was a great writer and he was eccentric and he was clever without being pretentious," he added.
"And he was the warmest of people to be with, he was a most lovely friend."
Fellow Python member John Cleese tweeted he was "utterly dismayed" to hear of Innes's death, calling him "a very sweet man, much too nice for his own good".
Utterly dismayed to hear about Neil Innes. Right out of the blue...— John Cleese (@JohnCleese) December 30, 2019
A very sweet man, much too nice for his own good
Lovely writer and performer. When he worked with Python on our stage show, I listened every night to "How sweet to be an Idiot" on the tannoy
Innes had been travelling to his home in France after an evening out with his wife and friends.
His family have asked for privacy "at this difficult time".
"It is with deep sorrow and great sadness that we have to announce the death of Neil James Innes on 29 December 2019," they said in a statement.
"We have lost a beautiful, kind, gentle soul whose music and songs touched the heart of everyone and whose intellect and search for truth inspired us all.
"He died of natural causes quickly without warning and, I think, without pain."
They went on: "His wife Yvonne and their three sons, Miles, Luke and Barney, and three grandchildren, Max, Issy and Zac, give thanks for his life, for his music and for the joy he gave us all."
The Essex-born star, who was raised in Germany, performed piano, guitar and vocals in The Bonzo Dog Doo-Dah Band - later renamed The Bonzo Dog Band - who had a hit in 1968 with I'm the Urban Spaceman. The track was co-produced by Apollo C. Vermouth - a pseudonym for Sir Paul McCartney.
The comic songwriter penned the track Death Cab For Cutie, which was used in The Beatles' 1967 film Magical Mystery Tour, and later inspired the US band of the same name.
In the 1990s he was given a writing credit on the Oasis track Whatever, after it was found Noel Gallagher had borrowed portions of his song How Sweet To Be An Idiot.
'The Seventh Python'
In the 1970s, Innes worked closely with British comedy collective Monty Python, and Palin recalled how during their live shows he would "bring the house down" performing the track with a half cut out plastic duck on his head.
Innes contributed sketches and songs like Knights of the Round Table and Brave Sir Robin, as well as appearing in their classic films The Holy Grail and Life of Brian.
He wrote and performed sketches for their final TV series in 1974 after Cleese temporarily left, and was one of only two non-Pythons to be credited as a writer, alongside The Hitchhiker's Guide to the Galaxy author Douglas Adams.
A film about Innes called The Seventh Python was made in 2008.
After the comedy group came to an end, Innes joined fellow Python Eric Idle on a new series, Rutland Weekend Television, which gave birth to their spoof band The Rutles - a pastiche of The Beatles.
In the band Innes played Ron Nasty, a character loosely-based on John Lennon.
According to Innes's wife Yvonne, he toured with the Fab Four tribute act The Bootleg Beatles last year.
Reacting to the news on Monday, screenwriter Charlie Brooker admitted he'd had The Rutles track Cheese and Onions "stuck in my head for approximately 40 years".
Actor and writer Stephen Fry tweeted that Innes' earlier band "the Bonzos were everything to me when I was a teenager."
So sad to hear of the death of @NeilInnes – the Bonzos were everything to me when I was a teenager. I bought the LP of Tadpoles first: "Tackle the toons you tapped your tootsies to on Thames TV's Do Not Adjust Your Set". A great songwriter, great voice, great man.— Stephen Fry (@stephenfry) December 30, 2019
V sad to hear Neil Innes, the brilliant comedian, musician, and Rutle, has died. Humming this today, having had it stuck in my head for approximately 40 years: https://t.co/tLQKs75WTS— Charlie Brooker (@charltonbrooker) December 30, 2019
Fellow comedy writer Danny Baker also paid tribute, posting the below picture, as well as a link to one of Innes' tracks, Friends at the End.
"A bit numb that my friend has gone so suddenly," wrote Baker. "He wrote a beautiful song summing up how many of us feel today. Of course he did..."
While Baby Driver and Spaced director Edgar Wright tweeted he would "raise a glass to the man".
"Forever a fan of The Bonzo Dog Doo Dah Band," he wrote, linking to their 1968 track Canyons Of Your Mind.
"Allow me to share a favourite clip of them in action (with a bonkers Innes guitar solo, too). RIP Neil."
'Loved every moment'
In an interview in 2012 Innes reflected on the many different phases of his career.
"The thing is it all happened very quickly," he said.
"The Bonzos lasted for just five years or so and in that time we paid off three managers and had no holidays. Then I dropped into the Python era but that seemed to be over in a flash, too.
"And similarly The Rutles were a brief phase... They were only intended to be a short-term, one-off gag because the timing seemed so right."
He added: "People were desperate to get the Beatles back together and a guy in America was offering them $20m [£15.2m] each for a reunion! It was quite absurd.
"And George Harrison, who by then was closely involved with the Pythons, felt something even sillier needed to be done. He loved every moment of The Rutles. The 1996 revival for our Archaeology album was even funnier really."
In the same interview he described joining the other members of Python onstage at Concert for George - a 2002 tribute to the late Beatles guitarist and Python collaborator George Harrison, at London's Royal Albert Hall - as "probably one of the most special evenings of my life."
Comedians Diane Morgan and Richard Herring, along with singer-songwriter Billy Bragg and broadcaster Gary Crowley all added their tributes on Monday, too. Morgan described Innes as "one of the nicest people I've ever met and a towering talent".
Fairwell Neil Innes. One of the nicest people I’ve ever met and a towering talent. pic.twitter.com/JJ2vZUtkKO— Diane Morgan (@missdianemorgan) December 30, 2019
"I've suffered for my music and now it's your turn" RIP Neil Innes https://t.co/o4WUn6FUej— Billy Bragg (@billybragg) December 30, 2019
Really sad to hear about Neil Innes. Punch in the gut to end the decade on. RIP to the sweetest of idiots.— Richard K Herring (@Herring1967) December 30, 2019