Officials can be heard laughing as Boris Johnson's spokeswoman Allegra Stratton jested about it being a "business meeting".
The recording, passed to ITV News, is of a rehearsal for planned daily TV media briefings that Ms Stratton was supposed to host, but which the PM later scrapped.
Ed Oldfield, adviser to the PM, said: "I've seen reports on Twitter that there was a Downing Street Christmas party on Friday night, do you recognise those reports?"
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