Rishi Sunak, Chancellor of the Exchequer, will also be fined for the party, which celebrated Johnson's birthday in June 2020.
The Metropolitan Police have issued 30 new fixed penalty notices, including to Johnson's wife, Carrie.
When news of the party broke, Conservative MP Conor Burns' defended the party saying the Prime Minister was 'ambushed with a cake".
The Conservative party has been dogged by allegations of numerous gatherings and parties during the nation's lockdowns.
The Met Police has already handed out 20 fixed penalty notices concerning some of these.
Johnson and Sunak have both said they would not step down, despite calls for their resignations.
"There was a brief gathering in the Cabinet Room shortly after 2 pm lasting for less than 10 minutes, during which people I work with kindly passed on their good wishes," he said.
"And I have to say in all frankness at that time it did not occur to me that this might have been a breach of the rules.
"I now humbly accept that I was.
"But I think the best thing I can do now is, having settled the fine, is focus on the job in hand. That's what I'm going to do."