The Supreme Court on Thursday dealt a significant blow to executive power, cutting back on the president’s power to issue recess appointments during brief breaks in the Senate’s work.

The court ruled unanimously that President Obama had violated the Constitution in 2012 by appointing officials to the National Labor Relations Board during a short break in the Senate’s work when the chamber was convening every three days in short pro forma sessions when no business was conducted. Those breaks were too short, Justice Stephen G. Breyer wrote in a majority opinion joined by the court’s four more liberal members.

Justice Breyer added that recess appointments remain permissible so long as they are made during a break of 10 or more days. But many experts say that if either house of Congress is controlled by the party opposed to the president, lawmakers can effectively block such appointments by requiring pro forma sessions every three days. Each house must get the approval of the other chamber for recesses of more than three days.

More: Supreme Court Curbs President’s Power to Make Recess Appointments