A federal judge on Monday struck down a $100,000 fee U.S. President Donald Trump imposed on new H-1B visas for highly skilled ‌foreign workers, concluding that it constituted an unlawful tax Congress never authorised.

U.S. District Judge Leo Sorokin in Boston issued the ruling in a lawsuit filed by 20 Democratic state attorneys general challenging a fee Trump announced in September that dramatically raised the cost of obtaining H-1B visas, which tech companies in particular rely heavily on to bring in foreign workers.

The administration argued that the fee constituted a lawful monetary penalty that the president was authorised to impose under federal immigration law, which gives him the power to restrict the entry of certain foreign nationals when he deems it "detrimental to the interests of the ⁠United States."

But Sorokin concluded that the fee was not a penalty but a tax that the Republican president lacked authorisation from Congress to issue, and that the U.S. State Department and U.S. Citizenship and Immigration Services could not implement.

"Here, the substance and application of the $100,000 payment reveal that ​it is a tax, regardless of what the payment is called," wrote Sorokin, who was appointed by Democratic President Barack Obama.

The judge cited the U.S. Supreme Court's February ruling striking down Trump's sweeping tariffs, which he pursued under a law meant for use in national emergencies.

Under the logic of the justices' decision in that case, Trump similarly had no ​authority under immigration law to levy a tax, Sorokin said.

White House spokeswoman Taylor Rogers, in a statement, said the Trump administration is confident Sorokin's ​order will be reversed on appeal.

"President Trump has clear legal authority to restrict entry of any class of aliens he determines is not in America’s best interests, and ‌that is ⁠exactly what he did," she said.

The H-1B program offers 65,000 visas annually, with an additional 20,000 for workers with advanced degrees, each approved for three to six years. Employers seeking a visa for a foreign worker before Trump's proclamation typically paid about $2,000 to $5,000 in fees, depending on various factors.

Trump, in imposing the hefty new fee, in a proclamation said the H-1B program "has been deliberately exploited to replace, rather than supplement, American workers with lower-paid, lower-skilled labour."

The fee does not apply to visas granted to foreign citizens already in the United States on student visas, who generally make up a large share of new H-1B recipients.

Few employers have paid Trump's fee since it was instituted. As of February 15, USCIS had received just 85 payments of the $100,000 fee, an agency ​official said in a March filing.

The Trump administration has also ordered enhanced vetting of H-1B applicants and proposed a ​new visa selection ⁠process that would favour higher-skilled and better-paid workers.

The $100,000 fee prompted at least three lawsuits challenging its implementation, including one by the U.S. Chamber of Commerce, which is appealing a December decision by a judge in Washington, D.C., who rejected its claim that Trump had no authority to set the fee.

California Attorney General ⁠Rob Bonta, ​a Democrat who led the multi-state coalition that filed the case before Sorokin, hailed his ​ruling for striking down Trump's "unlawful and costly $100,000 tax."

"This tax was an attack on America's ability to attract and retain the high-skilled talent that strengthens our economy and helps us meet critical ​workforce needs,” he said.