A federal judge in California has ordered the Trump administration to halt investigations in nine states into whether noncitizens are registered to vote.

US district court judge Araceli Martínez-Olguín in Oakland found the government's effort was relying on data that is full of errors and that it was leading to intimidation of groups performing voter assistance efforts. She said it should be halted until polls close on election day on 3 November.

She put her ruling, which was made on Wednesday, on hold until Friday to give the administration a chance to appeal.

The ruling applies to investigations in California, Connecticut, Georgia, Missouri, Nevada, New York, Pennsylvania, Washington and Wisconsin.

It comes as the Trump administration has been hunting for noncitizen voters throughout the country in the months leading up to the midterm elections. The administration has signaled that it is a widespread problem, when in fact it is quite rare, as even its own investigations have shown.

The lawsuit was filed by the California Nurses Association and National Nurses United, along with the Voto Latino Foundation, San Francisco and Santa Clara county.

In response to the ruling, the Department of Homeland Security issued an email statement on Thursday saying it was focused on "removing aliens who vote and dilute the votes of U.S. citizens", while adding: "We have no plans to question or investigate people we believe to be U.S. citizens." According to legal filings in the case, investigators were told to target naturalized citizens to see whether they had voted before they became US citizens. The lists of voters to check came from sources that included state and local governments and publicly available information. Martínez-Olguín called them "voluminous and unreliable".

"Even under the best conditions, there is a high likelihood of error in review because there is no federal database containing citizenship status of U.S. residents," Martínez-Olguín wrote in her ruling. "The time limitations implemented during the Surge further exacerbate the risk of error." The Department of Justice has been demanding detailed and unredacted voter roll data from the states and has sued the 30 states that have refused to turn the information over, losing each of those cases so far. The judge noted that for states that refused to share their records, the administration was trying to obtain them through "unreliable commercial versions of voter roll data".

Martínez-Olguín said during a "surge" phase of the investigation starting in late August, agents were ordered to complete reviews of flagged individuals taking no more than 12 minutes per person to assess whether they were legally qualified to vote.

In their challenge, the plaintiffs said some US citizens felt they were wrongly identified and investigated as part of the inquiry. Martínez-Olguín said those fears "are well understood as intimidation" that has chilled people from attending campaign events and working as poll monitors during the election.

Beau Tremitiere, a lawyer representing the unions and voter group, said in a statement that the ruling affirms the groups' position.

"It was clear from the start that DHS built its lists on bad data, never checked how often it was wrong, and recklessly sent agents to people's homes weeks before an election," Tremitiere said.

Details of the investigation, known as the "Unlawful Voter Initiative", came to light last month after Chuck Schumer, the Democratic Senate minority leader, of New York and Alex Padilla, the Democratic California senator, released a whistleblower report about it.