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

US District Judge Leo Sorokin in Boston issued the ruling, opens new tab 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.

The ⁠H-1B programme offers 65,000 visas annually, with another 20,000 visas ​for workers with advanced degrees, 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.

The increase in fees ​has discouraged H-1B visa requests, according to court filings. As ​of February 15, US Citizenship and Immigration Services had received just 85 ‌payments ⁠of the $100,000 fee, the administration said in a March filing.

The administration argued that the fee constituted a monetary penalty that the president had lawful authority to impose under federal immigration ​law to restrict ​the entry of ⁠certain foreign nationals.

But Sorokin, who was appointed by Democratic President Barack Obama, concluded that ​the fee was not a penalty but a ​tax that ⁠the Republican president lacked any authorization from Congress to issue.

“Here, the substance and application of the $100,000 payment reveal that it is ⁠a tax, ​regardless of what the payment is ​called,” he wrote.

The White House did not immediately respond to a request ​for comment.