Donald Trump has suspended for an initial six months the entry of foreign students seeking to study or participate in exchange programmes at Harvard University.
The US president issued the proclamation on Wednesday, citing "national security" concerns and declaring it "detrimental" to US interests to continue allowing foreign students at the institution.
Within hours, Harvard responded by filing court documents asking a judge to block the "retaliatory" order from taking effect.
Trump's announcement is a further escalation of an ongoing legal row with one of the most prestigious US universities after Harvard refused to yield to a series of White House demands in April.
Wednesday's order comes after a judge blocked the Department of Homeland Security (DHS) from banning international students at Harvard in a ruling last week.
