Experts in the skill said the man, who stood on stage next to world leaders including Barack Obama, just “made it up as he went along” in a series of nonsense gestures.
His bizarre antics confused millions of deaf TV viewers around the world who tuned in to watch the tribute to the former South African president, who died on Thursday.
It also raised major concerns over security at the service, which was attended by 91 world leaders including British PM David Cameron.
Jan Sheldon, chief executive of the UK’s Royal Association for Deaf People, said: “We consider this to be an absolute disgrace and an insult to deaf people. It represents one of the most public mockeries of deaf people and sign language that we’ve ever seen.”
Concerns were raised over the interpreter by deaf people watching the service held at Johannesburg’s FNB Stadium on Tuesday.