(Reuters) -A federal judge on Thursday said U.S. President Donald Trump's firing of a Democratic member of the National Labor Relations Board was illegal and ordered that she be reinstated to her post.

The decision by U.S. District Judge Beryl Howell in Washington, D.C., restores a quorum of three members at the labor board, which had been paralyzed and unable to decide cases involving private-sector employers after Trump removed Gwynne Wilcox in January.

Wilcox claimed that Trump violated a requirement in federal labor law that NLRB members only be removed for "neglect of duty or malfeasance in office." The Trump administration acknowledged violating the law, but said the protections from removal for board members were unconstitutional.

Howell in a 36-page decision sharply rejected those claims.

"An American President is not a king - not even an 'elected' one - and his power to remove federal officers and honest civil servants like [Wilcox] is not absolute, but may be constrained in appropriate circumstances, as are present here," wrote Howell, an appointee of Democratic President Barack Obama.

"The President's interpretation of the scope of his constitutional power - or, more aptly, his aspiration - is flat wrong," she added.

White House spokeswoman Anna Kelly called Howell "a radical liberal judge" and said her "clearly partisan opinion" would not be the last say on the matter.

"The Trump Administration will pursue all legal remedies to vindicate the President's power for removal," Kelly said in a statement.

Deepak Gupta, a lawyer for Wilcox, said that "this decisive victory firmly rejects an extreme presidential power grab."

He said the decision was a win not only for Wilcox, but also for the integrity of the NLRB and its mission to protect American workers.

The NLRB, which has five members when fully stocked, enforces laws protecting private-sector workers' rights to organize, join unions and advocate for better working conditions, and oversees union elections.

With Wilcox reinstated, the board will have two Democratic members and one Republican, Marvin Kaplan, whom Trump has appointed acting chair. That means the board can resume issuing decisions in individual cases, such as those claiming that workers were fired for supporting union campaigns or that companies changed working conditions without bargaining with unions.

The ruling was the latest instance of judges rebuffing Trump's attempts to remove officials from agencies like the NLRB that are designed to be independent from the White House.

Earlier this week, two judges separately ruled that Trump could not fire a member of the board that hears federal employees' appeals when they are fired or the head of the Office of Special Counsel, an agency watchdog that protects the rights of whistleblowers, although a federal appeals court later cleared the way for Trump to remove the latter.

The NLRB is typically made up of three members from the president's party and two from the opposing party. They serve staggered terms, so control of the board generally does not change hands for a year or two into a presidential administration.

About two dozen companies, including Amazon and Elon Musk's SpaceX, have filed lawsuits since last year challenging removal protections for board members and claiming the president should have the power to fire them at will. The companies also argue the NLRB's in-house enforcement procedures are unconstitutional.

Musk is a top adviser to Trump and is spearheading an effort to drastically shrink the federal bureaucracy and slash government spending.