LONDON — On Prime Minister Boris Johnson’s turbulent path to Downing Street, serial philandering and ethical sloppiness became part of his schtick, blots on a career so chaotic and beguiling that the British public always seemed to forgive the mistakes.
But Mr. Johnson is now facing a potentially more serious accusation of mixing friendship with a young woman and misspent public money, one that could test voters’ patience in a looming general election.
In an article published this weekend, The Sunday Times of London reported that, when Mr. Johnson was mayor of London, his office directed tens of thousands of pounds in government money to a fledgling entrepreneur and close friend whose apartment he often visited during working hours.
The entrepreneur, Jennifer Arcuri, an American and a former model, was 27 when she first crossed paths with Mr. Johnson in 2012. In the ensuing years, she was given coveted spots on trade missions with the mayor to Tel Aviv, New York, Singapore and Malaysia. In some instances, Mr. Johnson’s office intervened to add her to the roster even though she did not meet the criteria for trade delegates, The Sunday Times reported.
Even with lawmakers consumed by the fight over Brexit, the scandal gathered steam on Monday.
On a plane to the United Nations General Assembly in New York, Mr. Johnson repeatedly refused to answer reporters’ questions about the article, including whether he was in a sexual relationship with Ms. Arcuri at the time.
Jeremy Corbyn, the leader of the opposition Labour Party, accused the prime minister of an “alleged abuse of power.”
And the British Department for Digital, Culture, Media and Sport, which this year awarded Ms. Arcuri’s company a grant of 100,000 pounds, or about $125,000, intended for British firms, said it was investigating the grant after The Sunday Times reported that Ms. Arcuri had vacated the company’s registered address in England and now lives in California.
Mr. Johnson has previously weathered one scandal after another, building a buffer from his aura of chronic absent-mindedness and posh eccentricity. But he has spent considerable political capital in trying to enforce an Oct. 31 deadline for Brexit, and in the process provoked Parliament and the courts to try to check his power grab.
Analysts said that Mr. Johnson was left vulnerable for the first time in his two-decade political career.
“This goes into a new category — it’s no longer just the tabloid sex scandal thing,” said Sonia Purnell, the author of “Just Boris: A Tale of Blond Ambition.” “Every single ingredient there is alarming. And the fact that he will not answer any questions — or indeed deny any of those allegations — I think that is why some people see blood in the water.”
The scandal came in the midst of a tense week for the prime minister. He is trying to secure a new Brexit deal with European leaders. An early election is looming. And the Supreme Court is set to rule on Tuesday on whether Mr. Johnson’s decision to suspend, or prorogue, Parliament at the height of the Brexit crisis was legal.
That shutdown was originally intended to keep lawmakers from interfering with his plans to yank Britain out of the European Union by Oct. 31, with or without a deal governing future relations. But it now serves a second purpose in shielding the prime minister from a very public interrogation over the government grants.
“The fact he’s prorogued Parliament means no one can call him to account,” Ms. Purnell said.
As mayor, Mr. Johnson was barred from giving friends improper benefits. He was also bound to declare any private entanglements that could have created conflicts of interest.
Hacker House, a cybersecurity company set up by Ms. Arcuri, did not to respond to a message on Monday seeking comment. She told The Sunday Times that the grants “were purely in respect of my role as a legitimate businesswoman.”
The Sunday Times reported that Ms. Arcuri arrived in London for business school after a short career in modeling and films. Her first venture — a series of networking events for entrepreneurs and policymakers in East London — was given a lift by Mr. Johnson, who appeared at four of her events, The Sunday Times reported.
Soon a promotional agency overseen by the mayor began giving the company money. Mr. Johnson also paid regular visits to Ms. Arcuri’s East London apartment, which was outfitted with a dancing pole, sometimes during afternoon work breaks. The apartment building owner told The Sunday Times that Ms. Arcuri described Mr. Johnson as “one of her best friends.”
And after Ms. Arcuri was turned down for a spot on a mayoral trade mission to Tel Aviv, Mr. Johnson’s office asked for her to be added, even though her professional experience had little to do with the trip.
Mr. Johnson, who was mayor of London from 2008 to 2016, has occasionally mixed his elected positions with his dalliances before, as when he gave Helen Macintyre, an art adviser, an unpaid job as a fund-raiser for a public sculpture project. It was later revealed that Ms. Macintyre and Mr. Johnson had a child together.
Earlier in his career, Mr. Johnson was fired from the Conservative Party’s leadership team after falsely denying reports of an extramarital affair.
But these incidents only served to bolster Mr. Johnson’s image as a blustering and buffoonish, but always entertaining, lawmaker.
“He’s got some sort of hold on the British psyche,” Ms. Purnell said. “He’s created this persona: the toff who quotes Latin. And no one really asks him what he’s doing, whether it’s a good idea. That’s what got him this far.”
The Labour Party was already raising an alarm on Monday about parts of the British news media, particularly the British Broadcasting Corporation, giving Mr. Johnson a pass, though the B.B.C. denied the allegation.
“This is how the establishment works,” Mr. Corbyn said in a Twitter clip. “They close ranks.”