The Trump administration has abandoned its lawsuit seeking to force California to set aside mail-in ballots cast by voters federal officials alleged were likely non-citizens.

The Justice Department filed a notice of voluntary dismissal in federal court in Los Angeles on Friday, ending a fight that reached the Supreme Court before the midterm elections.

The lawsuit, filed on 15 October, sought to compel California and Los Angeles County election officials to separate mail ballots belonging to about 20,000 registered voters whom the federal government had identified as likely non-citizens. The ballots would have been held back until the voters' citizenship could be verified.

California challenged the list's accuracy, warning it included eligible voters and that complying could disenfranchise American citizens.

Four court defeats in a row

The administration lost at every stage. US District Judge David Carter denied emergency relief, and the 9th US Circuit Court of Appeals twice rejected its attempts to force officials to segregate the ballots. On 2 November, the day before the election, the Supreme Court also declined to intervene.

In Friday's filing, the department acknowledged the relief it sought was no longer available. A hearing was set for 13 November, but by then California officials would already have mixed the disputed ballots with other votes.

"There is then no way to get the challenged ballots segregated," the department's lawyers wrote, adding that the case was now moot.

California's attorney general, Rob Bonta, said the dismissal followed four consecutive court losses and came before a hearing that would have tested the government's list.

The department signalled it was not abandoning its wider efforts, saying it was evaluating how best to protect American votes, including by preventing vote dilution by non-citizens. The case ends without a ruling on the merits.