The National Labor Relations Board has been thrown into a strange legal limbo — with the possibility that more than 300 of its decisions over the last year could be nullified — as a result of a federal appeals court ruling on Friday that President Obama’s recess appointments to the board were invalid.

By ruling that Mr. Obama’s three recess appointments last January were illegal, the federal appeals court ruling, if upheld, would leave the board with just one member, short of the quorum needed to issue any rulings. The Obama administration could appeal the court ruling, but no announcement was made on Friday.

If the Supreme Court were to uphold Friday’s ruling, issued by the United States Court of Appeals for the District of Columbia Circuit, it would mean that the labor board did not have a quorum since last January and that all its rulings since then should be nullified.

Many Republicans and business groups applauded Friday’s ruling. They often assert that the appointments Mr. Obama made to the board have transformed it into a tool of organized labor. But many Democrats and labor unions say Mr. Obama’s appointments restored ideological balance to the board after it was tipped in favor of business interests under President George W. Bush