Migrant families who cross the southern border of the US illegally could be detained indefinitely under a new regulation announced by the Trump administration.
It replaces an agreement that set a limit on how long the government could hold migrant children in custody.
The move was announced by Acting Homeland Security Secretary Kevin McAleenan and is due to come into effect in 60 days.
It is expected to face legal challenge.
Homeland security officials believe that time limits on the detentions of migrant families have driven the surge of Central Americans crossing the border into the US this year.
They argue that the new regulation will counter the belief that bringing children into the US is "a passport" to being freed from detention after a short period.
"Today the government has issued a critical rule that will permit the Department of Homeland Security to appropriately hold families together and improve the integrity of the immigration system," said Mr McAleenan.
"This rule allows the federal government to enforce immigration laws as passed by Congress and ensures that all children in US government custody are treated with dignity, respect, and special concern for their particular vulnerability."
The American Civil Liberties Union (ACLU) condemned the policy, saying: "The government should not be jailing kids, and certainly shouldn't be seeking to put more kids in jail for longer."
It comes as the White House seeks to enact a "public charge" rule that would prohibit legal migrants from accessing social services such as food aid. The Trump administration has also recently moved to end asylum protections for most Central American migrants.
The new regulation aims to replace a decades-old court agreement - known as the Flores settlement - that both limited how long the government could hold migrant children in custody and specified the level of care they must receive.
A 2015 legal ruling on this issue specified that children should be held for no more than 20 days.
Under the new rules, the government could send families caught crossing the border illegally to family residential centres for the duration of their immigration cases.
President Trump has repeatedly complained about the "catch and release" of migrant families under the old rules.
Last summer, the Trump administration separated children from their parents as a means of circumventing the Flores settlement.
The children were held by the Department of Health and Human Services while the adults were imprisoned while awaiting trial for breaking immigration laws.
In June 2019, a Trump administration lawyer was admonished by federal judges in San Francisco after she argued that the Flores settlement did not require the government to provide detained children with soap or toothbrushes.