LONDON — Prime Minister Boris Johnson of Britain has escaped without additional penalties in a long-simmering scandal that threatened his grip on power, as the Metropolitan Police announced on Thursday that they had concluded an investigation into lockdown-breaking parties at Downing Street and Whitehall.
The police said they had imposed 126 fines, known as penalty notices, relating to eight social gatherings held in and around Downing Street, which combines office space and the prime minister’s official residence, during periods when the government restricted social mixing.
Mr. Johnson had already paid one fine for taking part in a birthday celebration for him, and he was known to have attended other gatherings that were investigated by the police, but he was not among those assessed with additional fines, according to a Downing Street spokesman.
The lack of any other penalties lifts a cloud that had hung over the prime minister since the reports of illicit parties first erupted in November last year. Mr. Johnson had faced a potential leadership challenge from members of his Conservative Party, but that danger appears to have receded.
The spotlight will now shift to the leader of the opposition Labour Party, Keir Starmer, who is under pressure for taking part during a lockdown in a beer-and-Indian-food gathering with members of his party that is being investigated by the police in Durham.
Mr. Starmer has said that if he receives a fine, he will step down as party leader — a move designed to increase the pressure on Mr. Johnson, who said he would stay in office, regardless of whether he was found to have broken the law.
The Labour Party harshly criticized Mr. Johnson on Thursday, with Angela Rayner, the party’s deputy leader, saying in a statement, “Industrial scale rule breaking has taken place at the heart of Boris Johnson’s Downing Street.”
But now it is Ms. Rayner who is under pressure. She was alongside Mr. Starmer at the gathering that is under investigation in Durham.
The police in London said that in imposing the fines, 53 went to men and 73 to women. Some people received more than one.
In addition to Mr. Johnson, his wife, Carrie Johnson, was informed that she would not face a fine. The chancellor of the Exchequer, Rishi Sunak, who was fined for attending a party, will also not receive any additional penalties, according to Downing Street.
“Our investigation was thorough and impartial and was completed as quickly as we could, given the amount of information that needed to be reviewed and the importance of ensuring we had strong evidence,” the police said in a statement.
The police said 12 detectives worked on the case, code-named Operation Hillman. They reviewed 345 documents, 510 photographs and surveillance footage, as well as collecting 204 questionnaires.
But the police’s role in the scandal has been sharply criticized. For months the London force declined to investigate reports of illicit parties at government offices, which emerged in daily media reports, only to reverse course and open an investigation in late January, just as a separate internal government investigation of the parties was about to be released.
That report, by a senior civil servant, Sue Gray, was published only in highly redacted form. With the police investigation completed, the government has pledged to release the full report. It is expected to cast a harsh light on the alcohol-fueled culture in Downing Street under Mr. Johnson’s leadership.