Mr. Carter continued the practice, which violated Defense Department rules, for at least two months after it was publicly revealed in March that Hillary Clinton had exclusively used a personal email account as secretary of state, the officials said.

It is not clear when Mr. Carter stopped using the account. But an administration official said that when the White House chief of staff, Denis R. McDonough, learned about Mr. Carter’s email practices in May, Mr. McDonough directed the White House Counsel’s Office to contact the Defense Department to ask why Mr. Carter was relying on the personal account…

In a written statement on Wednesday, a spokesman for Mr. Carter said that the defense secretary had determined that he had been wrong to use the personal account.

“After reviewing his email practices earlier this year, the secretary believes that his previous, occasional use of personal email for work-related business, even for routine administrative issues and backed up to his official account, was a mistake,” said the spokesman, Peter Cook. “As a result, he stopped such use of his personal email and further limited his use of email altogether.”