WASHINGTON — On any given day, about 300 immigrants are held in solitary confinement at the 50 largest detention facilities that make up the sprawling patchwork of holding centers nationwide overseen by Immigration and Customs Enforcement officials, according to new federal data.

Nearly half are isolated for 15 days or more, the point at which psychiatric experts say they are at risk for severe mental harm, with about 35 detainees kept for more than 75 days.

While the records do not indicate why immigrants were put in solitary, an adviser who helped the immigration agency review the numbers estimated that two-thirds of the cases involved disciplinary infractions like breaking rules, talking back to guards or getting into fights. Immigrants were also regularly isolated because they were viewed as a threat to other detainees or personnel or for protective purposes when the immigrant was gay or mentally ill.

The United States has come under sharp criticism at home and abroad for relying on solitary confinement in its prisons more than any other democratic nation in the world. While Immigration and Customs Enforcement places only about 1 percent of its jailed immigrants in solitary, this practice is nonetheless startling because those detainees are being held on civil, not criminal, charges. As such, they are not supposed to be punished; they are simply confined to ensure that they appear for administrative hearings.