Police sent questionnaires to more than 100 people, including the Prime Minister, and interviewed witnesses as part of the investigation.
The "partygate" scandal had left Johnson's tenure precarious before Russia launched a war in Ukraine more than a month ago that gave Britain's politicians more urgent priorities.
Johnson's grip on power was shaken by public anger over revelations his staff held "bring your own booze" office parties, birthday celebrations and "wine time Fridays" in 2020 and 2021 while millions in Britain were barred from meeting with friends and family because of strict rules to contain the spread of the virus.
Johnson has denied any wrongdoing, but he is alleged to have been at several of the dozen events in his 10 Downing St office and other government buildings that are being investigated by the police.
He has acknowledged attending a "bring your own booze" party in the Downing St garden in May 2020 during the first lockdown, but insisted he believed it would be a work event.