(Reuters) - President Donald Trump has removed a Democratic member of the U.S. National Labor Relations Board from office, an unprecedented move that will escalate an ongoing legal battle over the scope of the president's powers to control federal agencies.

Gwynne Wilcox, who was appointed to the board by Democratic former President Joe Biden, in a statement called her firing late Monday illegal and said she would pursue "all legal avenues" to challenge it.

The NLRB has five members appointed by the president, and before Wilcox was fired it had two vacancies. Without her, the board will lack a quorum and cannot issue decisions even in routine cases accusing companies or unions of violating federal labor law.

Once board members are confirmed, federal law allows them to be removed only for "neglect of duty or malfeasance in office." It was not immediately clear whether Trump gave a reason for terminating Wilcox.