The broadcaster said it was alleged the prime minister's wife, Carrie Johnson, helped organised the party on the afternoon of June 19.
Up to 30 people attended the event in the Cabinet Room of Number 10, his office and residence, ITV said.
The report adds to the pressure on Johnson over a series of gatherings at Number 10 that would seem to have broken the COVID-19 lockdown rules imposed by his government.
Senior bureaucrat Sue Gray has been investigating the parties and is expected to publish a report later this week.
In response to the birthday party allegation, Johnson's office told ITV: "A group of staff working in No 10 that day gathered briefly in the Cabinet Room after a meeting to wish the Prime Minister a happy birthday. He was there for less than ten minutes."