"Our investigation into the Conservative Party found that the laws around the reporting and recording of donations were not followed," Louise Edwards, director of Regulation at the independent body which oversees elections and regulates political finance in the UK, said.
The commission said that the party failed to fully report a donation of $90,000 made by a private donor in October 2020, a large part of which was connected to the costs of refurbishment to Johnson's apartment.
The full value of the donation was not reported as required in the party's Q4 2020 donation report, the election watchdog said.
In April, before the investigation began, Johnson claimed that he had paid for the revamp of his Downing Street residence from his own pocket.