The Manhattan district attorney’s office will halt the prosecution of Tracy McCarter, a woman charged with murder in 2020 after her estranged husband died in a confrontation with her, according to a letter sent to the judge presiding over the case.

Ms. McCarter’s lawyers have maintained that she did not stab her husband, James Murray, but that minutes after he choked her, she held a knife in a defensive posture and he charged. Her case has become a cause célèbre for activists seeking to draw attention to the perils of domestic violence.

The Manhattan district attorney, Alvin L. Bragg, had tweeted in support of Ms. McCarter while he was running for the office. But activists were furious when, upon being sworn in, he continued with the prosecution.

In Friday’s letter to the State Supreme Court judge, Diane Kiesel, which was shared with The New York Times by Ms. McCarter’s defense team, Mr. Bragg said that he could not allow the case to go forward. He said that he had “reasonable doubt of whether Ms. McCarter stabbed Mr. Murray with the requisite intent to support a conviction of murder in the second degree.”

He wrote that he could not in good conscience “allow a prosecution to proceed to trial and ask a jury to reach a conclusion that I have not reached myself.”

Mr. Bragg “righted a grievous injustice,” said Sean Hecker, a lawyer for Ms. McCarter, in a statement.

“Tracy McCarter is an innocent survivor of domestic violence who has suffered mightily from a criminal justice system that demands change,” he said.

A spokeswoman for the district attorney’s office declined to comment, other than to say the next court appearance in Ms. McCarter’s case is Nov. 28.

It is highly unusual for a prosecuting office to say that it will not move forward with a murder case after a defendant has been charged, and the move may renew criticism of Mr. Bragg from skeptics who say that his office has been too lenient. The Republican candidate for governor, Lee Zeldin, had pledged to remove Mr. Bragg from office on the basis of that criticism before losing to his Democratic rival, Gov. Kathy Hochul.

Mr. Bragg’s office had attempted to reduce the charges against Ms. McCarter. In May, prosecutors pursued an unorthodox arrangement under which Ms. McCarter would have pleaded guilty to second-degree manslaughter and to menacing, while not admitting responsibility. Under the terms of that deal, Ms. McCarter would have been allowed to have the manslaughter plea vacated after a year.

But Justice Kiesel said that plea arrangement was barred by law and rejected it. And in August, she denied prosecutors’ more straightforward request to dismiss the indictment and charge Ms. McCarter with manslaughter. She instead ordered a trial on the murder charge.

That left Mr. Bragg with few choices, he said in his letter.

“At this stage — with the proposed plea and reduced charge foreclosed — the options remaining available to me as district attorney are stark: to proceed or decline to proceed to trial on a charge where I have reasonable doubt. Given those options, I decline to proceed with prosecution of the indictment.”

As his letter acknowledged, Mr. Bragg does not have the power to dismiss the indictment himself. He recommended that the judge do so, though it is unclear how she will respond.