Britain's leader was overheard telling former New York Mayor Michael Bloomberg that the monarch appeared relieved that the Scots voted to stay in the United Kingdom.
Microphones picked up an unguarded Cameron describing how the queen "purred down the line" after hearing the results.
Conversations with the monarch are considered private and her views are rarely aired. Cameron said Wednesday he was sorry.
"It was a private conversation, but clearly a private conversation that I shouldn't have had and won't have again," he said. "My office has already been in touch with the palace to make that clear and I will do so as well."