Washington DC - A federal judge barred the US administration from using a central database to verify voter rolls, ruling that the government violated privacy laws by combining personal data from millions of US citizens to help states check the citizenship status of registered voters.

The case centers on the Systematic Alien Verification for Entitlements (SAVE) system, which was originally designed to verify immigration and citizenship status for government benefits. The Trump administration expanded the system by incorporating additional data, including records from the Social Security Administration, to allow states to screen voter rolls for possible non-citizens.

In her ruling, which was published on Monday, the judge said the administration had rushed the rollout of an error-prone system despite knowing it could incorrectly identify eligible voters as non-citizens. Naturalized US citizens could be particularly affected because some records were incomplete or outdated, she said.

The judge wrote that the federal government had knowingly disregarded the privacy rights of US citizens and put voting rights at risk.

According to the ruling, some states were already using the system to review voter registrations, resulting in eligible US citizens being incorrectly flagged as non-citizens and, in some cases, removed from voter rolls.

The decision is a setback for Trump's efforts to expand federal oversight of elections ahead of November's congressional midterm elections, when Republicans will seek to defend narrow majorities in both chambers of Congress.

Trump has long pushed for stricter proof-of-citizenship requirements for voters, arguing they are needed to prevent voting by non-citizens. Such cases are rare, however, and voting by foreign nationals is illegal.

The lawsuit was brought by voting rights and privacy groups. The Department of Homeland Security criticized the ruling and said it would challenge the decision.