US District Judge Victor Marrero on Wednesday ruled the draconian law - a cornerstone of the Bush administration's "war on terror" - violated constitutional prohibitions against unreasonable searches.
The Patriot Act granted "unchecked power" to the FBI to demand confidential customer records from communication companies, such as Internet service providers or telephone companies.
The provisions of the Act had been challenged by the American Civil Liberties Union (ACLU). Although the ACLU's suit was filed on behalf of an Internet access firm, the ruling could apply to other entities that have received FBI secretive subpoenas, known as national security letters.
The ACLU said the Patriot Act provision was worded so broadly that it could effectively be used to obtain the names of customers of websites such as Amazon.com, or a political organisation's membership list, or even the names of sources that a journalist has contacted by e-mail.
"This is a landmark victory against the Justice Department's misguided attempt to intrude into the lives of innocent Americans in the name of national security," said ACLU Executive Director Anthony Romero.
The September 11 attacks gave
birth to the Act
"Even now, some in Congress are trying to pass additional intrusive law enforcement powers. This decision should put a halt to those efforts," he said.
The FBI has had power to issue national security letters demanding customer records from communication companies since 1986, but only if the subject was suspected of being a foreign spy.
A section of the Patriot Act – a controversial law the Bush administration pushed through the Congress after the September 11 attacks to help it battle terrorism – gave the FBI more powers to obtain information through subpoenas.