Freeman, who will star as Bilbo Baggins in the two-part fantasy film, accidentally revealed the casting news backstage at the British Academy of Film and Television Arts (BAFTA) TV Awards on Sunday.
Early reports suggest Cumberbatch will voice the dragon character Smaug but the actor refused to specify his role, insisting, "I can't say at the moment, thank you very much."
A red-faced Freeman later told reporters, "I knew I was a sort of a big mouth but I didn't realize to what extent I was a big mouth, and I've just ruined everything. I might not actually go back to a job - they might have just sacked (fired) me."
Cumberbatch plays Sherlock Holmes opposite Freeman as Dr. Watson in a TV adaptation of Sir Arthur Conan Doyle's legendary sleuth stories. The duo picked up the Best Drama Series prize for the show at the BAFTA TV Awards.