David McCallum has died at the age of 90, it has been confirmed.
The actor was best known for his role in The Man from UNCLE back in the 1960s but more recently, he's best known for playing Donald 'Ducky' Mallard in CBS drama, NCIS.
He died in New York on Monday and his cause of death has been put down to natural causes.
Tributes from far and wide have flooded in for the actor, including from a number of his NCIS co-stars.
Michael Weatherly penned on Twitter: "David McCallum made every moment count, in life and on set.
"Let’s raise a jug and celebrate a funny fantastic authentic man. I’ve only got 3 autographs. Connery, Tony Bennett and McCallum. I felt the same way as Steve McQueen in this picture from The Great Escape: Wow! It’s David McCallum!
"No one did it better. We were lucky to have him bring us Ducky. Let’s send all the love in the world to his beautiful family. Rest In Peace David."
David McCallum has starred on-screen for over seven decades
While another former NCIS alum, Pauley Perrette, weighed in on Instagram: "Oh David. What a life. What a legend. What a journey. And your legacy. Will always be the love you have for your family.
"Love to Katherine and the kids and the grandkids, your absolutely very favorite things."
Elsewhere, current NCIS star Brian Dietzen penned a lengthy tribute: "A life well lived. David McCallum was a mentor, a great scene partner, a wonderful father and husband, and he was my dear friend.
"I met David on my first day of work at NCIS. I was booked to work one day on the show, but David and I hit it off. Our scene turned out well and they invited me back for more scenes with this screen legend.
"Several more. Over the course of the next 20 years, David and I (along with our alter-ego’s Ducky and Jimmy) developed a deep friendship and love of our shared work. He was a joy to work with. Always had a smile. Always had something new to add to the page, and ALWAYS had the ability to steal every scene he was in!
"He had a quick and wry sense of humor and was generous with advice when I asked for it. He was also quick to share praise for work well done. I loved getting a phone call from him after a show aired to hear him talk about the scenes (even as he appeared less and less in our show).
"The friendship and respect people saw between Ducky and Jimmy over the years mirrored the relationship we’d developed off camera. Whether it was going on field trips for character research, attending charity events or catching some golf together, he was always up for an adventure. The guy could do anything.
"He lived to be 90 years old, was a part of multiple hit tv shows, movies, broadway shows, and wrote a couple novels, all while raising a loving family. Thank you David. Love you my friend. I’ll miss you. Sending all my love to the McCallum family today."
David McCallum shot to fame for his Man from UNCLE role in the 1960s
Robert Wagner also took to Instagram to pen: "The world lost a great talent today. David was a brilliant actor who I had the pleasure of working with on several different projects. Thoughts to his wife, Katherine and to his sons and daughter."
And current star of the CBS drama Wilmer Valderrama commented: "David, what an immense honor it was to share the screen with you. Your professionalism and ability to effortlessly take every one of us on a journey through your art will be felt forever. I will miss you my friend."
McCallum's CV boasted a number of other high-profile credits besides The Man from UNCLE and NCIS.
He also appeared in The Great Escape, The Greatest Story Ever Told, and A Night to Remember on the big screen as well as TV shows Perry Mason and The Outer Limits.