BOSTON (Reuters) -A U.S. judge on Wednesday allowed Donald Trump's administration to move ahead with a buyout program for federal workers that could usher tens of thousands of workers off government payrolls as part of a sweeping effort to downsize the U.S. civil service. 

The decision by U.S. District Judge George O'Toole in Boston could clear the way for the Trump administration to swiftly wrap the program up, after the legal challenge had delayed its initial February 6 deadline for workers to accept the offer. The administration said the program is now closed to new applicants.

"There is no longer any doubt: the Deferred Resignation Program was both legal and a valuable option for federal employees," the Office of Personnel Management said in a statement.

Labor unions that brought the lawsuit to stop the plan did not immediately say whether they would appeal the judge's decision or pursue other options.

"Today's ruling is a setback in the fight for dignity and fairness for public servants. But it's not the end of that fight," said Everett Kelly, president of the American Federation of Government Employees, which represents 800,000 federal workers.

The buyout is one of many approaches Trump is taking to slash a civilian workforce of 2.3 million that he has blasted as ineffective and biased against him. He has also ordered government agencies to prepare for wide-ranging job cuts, and several have already begun to lay off recent hires who lack full job security.

As of Friday, about 65,000 federal employees had signed up for the buyouts, according to a White House official.

Unions urged their members not to accept the buyout and have warned that Trump cannot be trusted to honor it.

The offer promises to pay employees their regular salaries and benefits until October without requiring them to work, but that may not be ironclad. Current spending laws expire on March 14 and there is no guarantee that salaries would be funded beyond that point.

Lawyers with the U.S. Department of Justice had described the initiative as a "humane off-ramp" for those frustrated by Trump's broader plans to reduce the size of the workforce and end the ability of many to work from home.

Unions representing federal employees argued in their lawsuit that the program was "stunningly arbitrary" and violates a law that prevents agencies from spending more money than approved by Congress.

But O'Toole ruled they did not have standing to bring the lawsuit because they would not be directly affected by the program. Disputes involving federal employees also must first be pursued through an administrative process before going to court, he wrote in his decision.

Unions and Democratic attorneys general have brought several other lawsuits challenging Trump's rapid remaking of government and won some initial victories.

In a separate lawsuit filed on Wednesday, five unions sued to block what they called a possible mass firing of hundreds of thousands of workers who resist pressure to accept the buyouts.