Judicial independence is under grave threat on several fronts, Chief Justice John G. Roberts Jr. wrote on Tuesday in an unusually urgent and somber year-end report on the state of the federal judiciary.

“Violence, intimidation and defiance directed at judges because of their work undermine our Republic, and are wholly unacceptable,” he wrote.

The report, which arrived in the wake of questions about the court’s ethical standards and a drop in its approval ratings, said some criticism of judges’ work is healthy, warranted and welcome.

“Unfortunately, not all actors engage in ‘informed criticism’ or anything remotely resembling it,” he wrote. “I feel compelled to address four areas of illegitimate activity that, in my view, do threaten the independence of judges on which the rule of law depends.”

One, he wrote, was “violence directed at judges for doing their jobs.” The number of hostile threats and communication directed at judges has more than tripled in the past decade, he wrote. “In extreme cases,” he added, “judicial officers have been issued bulletproof vests for public events.”

At a judicial conference in September, Justice Amy Coney Barrett said she had been given one, prompting questions from her 13-year-old son.