Michael T. Flynn, the national security adviser, resigned on Monday night after it was revealed that he had misled Vice President Mike Pence and other top White House officials about his conversations with the Russian ambassador to the United States.

Mr. Flynn, who served in the job for less than a month, said he had given “incomplete information” regarding a telephone call he had with the ambassador in late December about American sanctions against Russia, weeks before President Trump’s inauguration. Mr. Flynn previously had denied that he had any substantive conversations with Ambassador Sergey I. Kislyak, and Mr. Pence repeated that claim in television interviews as recently as this month.

But on Monday, a former administration official said the Justice Department warned the White House last month that Mr. Flynn had not been fully forthright about his conversations with the ambassador. As a result, the Justice Department feared that Mr. Flynn could be vulnerable to blackmail by Moscow.

In his resignation letter, which the White House emailed to reporters, Mr. Flynn said he had held numerous calls with foreign officials during the transition. “Unfortunately, because of the fast pace of events, I inadvertently briefed the vice president-elect and others with incomplete information regarding my phone calls with the Russian ambassador,” he wrote. “I have sincerely apologized to the president and the vice president, and they have accepted my apology.”