Eagle-eyed fans of HBO’s lavishly produced fantasy epic poked fun at the show when a rogue cup was spotted on a table during an episode in the final season earlier this year.
Clarke – who played Daenerys Targaryen – was among those mentioned as a possible culprit as the cup was in front of her at the time.
Now, the actress, 33, has revealed co-star Conleth Hill, who portrayed master of spies Lord Varys, has admitted to being responsible for the error.
During an appearance on Jimmy Fallon’s US talk show, she said: “We had a party before the Emmys recently and Conleth, who plays Varys, whose sitting next to me in that scene, he pulls me aside and he’s like, ‘Emilia, I have got to tell you something, love. The coffee cup was mine’.”
Clarke added: “It was his! It was Conleth’s coffee cup.
“He said so. He said ‘ I think it was, I am sorry, darling, I didn’t want to say anything because it seemed the heat was very much on you.’
“I was like ‘what!? what!?”‘
Pressed by Fallon if she knew for sure it was Hill, Clarke said: “I think that’s who did it. He said it, he might have been drunk but he said it.”
The cup was originally identified as being from Starbucks but later confirmed to be from a local coffee shop in Banbridge, Northern Ireland, where much of Game Of Thrones was filmed.
It appeared during episode four of the show’s eighth and final season in May.
HBO later removed the cup from re-runs of the episode.
It was not the only anachronism of Game Of Thrones’ swansong, which received a lukewarm reception from fans and critics. A plastic water bottle was spotted in a key scene of the final episode.
Additional reporting by Press Association