LONDON — Prime Minister Johnson agreed to resign Thursday after days of defections crippled the controversial leader and left him unable to govern.

Mr. Johnson finally agreed to step down after one of his closest allies, Treasury chief Nadhim Zahawi, told the prime minister to resign for the good of the country. It was not immediately clear whether Johnson will stay in office while the Conservative Party chooses a new leader, who will automatically become prime minister as well. A formal announcement is expected later Thursday.

“Prime Minister: this is not sustainable and it will only get worse: for you, for the Conservative Party and most importantly of all the country,’’ Mr. Zahawi said in a letter to Johnson. “You must do the right thing and go now.”

Mr. Zahawi’s intervention came after two more members of Mr. Johnson’s Cabinet resigned, along with three junior officials, pushing the number of those who have left the government this week past 50.

Mr. Johnson on Wednesday rejected calls to resign, arguing that he had a mandate from the voters to remain in office. But by Thursday morning the man who has built a reputation for wriggling out of political controversies was forced to admit the reality of his situation.

A senior Conservative Party lawmaker, Bernard Jenkin, said he met with Mr. Johnson on Wednesday and advised him to stand down.

“I just said to him, ‘Look, it’s just when you go now, and it’s how you go. You can go with some dignity or you can be forced out like Donald Trump, clinging to power and pretending he’s won the election when he’s lost,’’ Mr. Jenkin told the BBC before Mr. Johnson agreed to resign.

Mr. Johnson, 58, managed to remain in power for almost three years, despite allegations that he was too close to party donors, that he protected supporters from bullying and corruption allegations, and that he misled Parliament and was dishonest to the public about government office parties that broke pandemic lockdown rules.

But recent disclosures that Mr. Johnson knew about sexual misconduct allegations against a Conservative lawmaker, Chris Pincher, before he promoted Mr. Pincher to a senior position turned out to be the last straw.

Last week, Mr. Pincher resigned as deputy chief whip after complaints he groped two men at a private club. That triggered a series of reports about past allegations leveled against Mr. Pincher — and shifting explanations from the government about what Mr. Johnson knew when he tapped him for a senior job enforcing party discipline.

The health secretary, Sajid Javid, and Treasury chief Rishi Sunak resigned within minutes of each other Wednesday over the scandal. The two Cabinet heavyweights were responsible for tackling two of the biggest issues facing Britain — the cost-of-living crisis and Covid.

Mr. Javid captured the mood of many lawmakers when he said Mr. Johnson’s actions threatened to undermine the integrity of the Conservative Party and the British government.

“At some point we have to conclude that enough is enough,” he told fellow lawmakers Wednesday. “I believe that point is now.”

Mr. Johnson had attempted to defy the mathematics of parliamentary government and the traditions of British politics. It is rare for a prime minister to cling to power in the face of this much pressure from his Cabinet colleagues.

The closest parallel may be Margaret Thatcher, the long-time Conservative prime minister who in 1990 sought to remain in office after her authority was undermined by disagreements over Britain’s relationship with what is now known as the European Union. Even she decided to resign after a number of Cabinet ministers told her it would be better for the party if she stepped aside.

Mr. Johnson may try to remain in office until the Conservative Party chooses a new leader, a process likely to take place over the summer. But some Conservatives said he should leave 10 Downing Street immediately to end the chaos engulfing the government.