The UK prime minister has been told by London's Metropolitan Police that he will be fined over COVID-19 lockdown breaches, a spokesperson for Johnson's office said. The prime minister said that he has already paid the fine.
The chancellor of the exchequer, or the government's finance minister, Rishi Sunak is also set to receive a fixed penalty notice, the official said.
"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. But of course the police have found otherwise and I fully respect the outcome of their investigation," Johnson told the BBC.
"Let me say immediately that I've paid the fine and I once again offer a full apology," he added.
Police had earlier said they would issue more than 50 fines to people who attended gatherings at Johnson's offices and residence. The parties took place at a time when such events were banned under coronavirus lockdown rules.
The allegations had threatened Johnson's position earlier this year when lawmakers in his own Conservative Party called for him to quit amid plummeting trust and popularity for the government.
Although Russia's invasion of Ukraine calmed that initial outcry, the threat to Johnson's premiership could resurface after news of the fine. Conservative members of parliament had said they would reserve judgment until the outcome of the police investigation.
Opposition demands resignations
The leader of the UK's main opposition Labour Party, Keir Starmer, called on both men to resign.
"Boris Johnson and Rishi Sunak have broken the law and repeatedly lied to the British public," Starmer tweeted. "They must both resign."
The criminal investigation followed numerous allegations and complaints that the police force had previously declined to investigate.
One of the parties — a birthday celebration for Johnson himself — allegedly took place during the first COVID-19 lockdown in June 2020. At that time, social gatherings indoors were banned.
Another two parties were held during a period of national mourning on the eve of the funeral of Prince Philip, the husband of Queen Elizabeth II for 73 years.
Johnson had denied any wrongdoing, despite being reported to have been present at several of the dozen events in his 10 Downing Street office and other government buildings.
Both men under fire
Revelations about the gatherings in 2020 and 2021, while millions of Britons were barred from meeting with friends and family, had stirred public outrage.
Thousands of people were fined between 60 pounds (€72, $78) and 10,000 pounds by police for breaking the rules around social gatherings.
A spokesperson for the prime minister's wife Carrie said she had also been notified by police that she would be fined over the breaches.
Sunak also recently came under fire after it was revealed that his wife Akshata Murty benefits from a non-domicile tax status. He was criticized for raising taxes during a cost-of-living crisis, while his family protect their dividends from UK taxation.