The US Supreme Court has ruled that President Barack Obama acted illegally when he made recess appointments to a key government agency, a decision that appears to block any president's ability to get around political gridlock in Washington by making appointments without congressional approval.

The court's unanimous decision on Thursday was the first case involving the US Constitution's recess appointments clause, and the nine justices held that Obama's appointments to the National Labor Relations Board in 2012 without Senate confirmation were illegal.

Obama had invoked the Constitution's provision giving the president the power to make temporary appointments when the Senate was in recess.

The court held, however, that the Senate was not formally in recess because it was briefly opened for business, for minutes only, every three days. The court said the Senate was only in recess if it was closed for 10 or more days.

Obama had argued that the Senate was on an extended holiday break and that those brief sessions every third day were a sham intended to prevent him from filling seats on the NLRB.

But the court, in a companion 5-4 decision, rejected a lower court's ruling that would have made it virtually impossible for a president to fill vacancies when the Senate is not transacting business. Recess appointments are good for only two years, making them a temporary tactic for a president who is unable to win Senate approval for his personnel decisions.

Potential importance

The issue of recess appointments receded in importance after the Senate's Democratic majority changed the rules to make it harder for Republicans to block confirmation of most Obama appointees.

But the ruling's impact may be keenly felt by the White House next year if Republicans capture control of the Senate in the November election. The potential importance of the ruling lies in the Senate's ability to block the confirmation of judges and the leaders of independent agencies like the NLRB.

A federal law gives the president the power to appoint acting heads of Cabinet-level departments to keep the government running.

The lower court ruling that was rejected by the Supreme Court had held that the only recess recognised by the Constitution was the once-a-year break between sessions of Congress. It also said that only vacancies that arise during that recess could be filled. So the high court has left open the possibility that a president, with a compliant Congress, could make recess appointments in the future.

Recess appointees who subsequently won Senate confirmation include Chief Justice Earl Warren and Justice William Brennan, Federal Reserve Chairman Alan Greenspan and two current NLRB members. Former UN Ambassador John Bolton is among recess appointees who left office because they subsequently could not win a Senate vote.