The prime minister confirmed he had paid the fixed penalty notice (FPN) for going to the hour-long gathering in the Cabinet Room on 19 June 2020.
Chancellor Rishi Sunak has also been fined, while the prime minister's wife Carrie Johnson confirmed she had paid an FPN relating to the same event.
Johnson apologised but said he would not resign over the incident.
"I accept in all sincerity that people had the right to expect better," he said in an interview at his Chequers country retreat.
