A Trump administration initiative to impose $100,000 fees on employers seeking visas for skilled foreign workers amounts to an unlawful tax on those companies and must be voided “in its entirety,” a federal judge ruled on Monday.

The decision by Judge Leo T. Sorokin of the Federal District Court for the District of Massachusetts nullified one of a series of tactics the Trump administration has used to restrict legal immigration, even in fields in which foreign skilled labor helped address severe shortages.

In a 42-page opinion, Judge Sorokin acknowledged that the policy, imposed in September, appeared to step on Congress’s “exclusive power” to levy taxes under the Constitution. He dismissed claims by the Trump administration that the fee was a “regulatory payment” that would have been within the executive branch’s power to set, not a tax.

“This is mere ipse dixit,” he wrote, meaning offered without evidence. “Defendants offer no definition for what constitutes ‘a regulatory payment,’ cite no cases or statutes employing the term, and advance no reasoned argument explaining how this term encompasses something different than a tax or a penalty.”

Judge Sorokin wrote that the rule was hastily formulated with no formal process or request for public comment, despite what might have been broad opposition to the rule across many industries that have historically relied on the visa program to fill critical needs.

The Trump administration had argued in filings that the H-1B program “has been deliberately exploited to replace, rather than supplement, American workers with lower-paid, lower-skilled labor.” Mr. Trump said the $100,000 fee would incentivize companies to hire more U.S. citizens into high-paying roles.