The actor made the revelation while appearing on the unscripted reunion special Harry Potter 20th Anniversary: Return To Hogwarts.
In the hit fantasy franchise, Radcliffe played Harry Potter, while Bonham Carter portrayed Bellatrix Lestrange.
During the special, Radcliffe said that he sent his co-star a note after wrapping up filming on the final Harry Potter film, The Deathly Hallows Part II, which was released in 2011.
Reading the message aloud, Radcliffe said: “Dear HBC, it was a pleasure being your co-star and coaster in the fact I always ended up holding your coffee.
“I do love you and I wish I’d just been born ten years earlier so I might’ve been in with a chance.”
Elsewhere on the documentary, Emma Watson described the moment she “fell in love” with Draco Malfoy actor Tom Felton, and other members of the cast reminisced about the time when Watson nearly quit the franchise.
It has also been confirmed that Harry Potter author JK Rowling will feature in the documentary, despite early reports that she would be absent following the backlash to her controversial comments about transgender people.
The special arrives in the UK on Sky and Now TV on New Year’s Day.
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