The US Supreme Court has rejected President Donald Trump's plan to restrict voting by mail before November's midterm elections, in a setback for the White House.

The justices upheld a federal judge's order that temporarily blocked the US Postal Service (USPS) from moving forwards with new requirements for postal ballots.

But it is unlikely to be the final word on the matter. Justice Brett Kavanaugh, a conservative, agreed with the ruling, but indicated he might later rule in Trump's favour as litigation in the case continues.

Trump, who argued the rules would combat electoral fraud, signed an executive order in March directing the USPS to deliver ballots only to voters on lists of citizens.

Legal challenges argued that the move violated states' constitutional rights to run elections.

Justice Kavanaugh wrote in the opinion that there was "at least a fair prospect that the final rule falls within the Postal Service's statutory authority". But he argued that "applying the rule in the 2026 elections would be arbitrary and capricious".

Justices Samuel Alito and Clarence Thomas, also conservatives, dissented. They wrote that the challenge to Trump's order was a "Hail Mary pass" that was ultimately unlikely to succeed.

"The Postal Service has broad authority to regulate the mail," they added.

The White House did not immediately comment on the decision.

About 30% of voters, mostly Democrats, cast postal ballots in the general election two years ago.

Since returning to the White House, Trump has sought to restrict voting by mail. While he himself has voted by post, he has long claimed this method allows non-citizens to cast ballots and is prone to fraud.

In March's executive order, the president directed USPS to introduce unique barcodes on postal ballot envelopes.

State and local election officials would also be required to supply information to an online portal about voters who receive mail ballots.

Twenty-three states and Washington DC sued to stop Trump's order.

The postal workers' union also said clerks could not be trained in time for the election.

In court filings, the Trump administration maintained the new USPS rules were constitutional.

It said states "cannot choose to use the federal mails to carry out their elections but then insist that their election-related mail is somehow exempt from the Postal Service's rule-making authority, conferred by Congress".

Boston-based Judge Indira Talwani imposed an injunction on 4 September blocking Trump's order. She ruled that implementing the measures close to November's elections could disenfranchise voters.

Last week, a US appeals court refused to put Judge Talwani's order on hold.

On Sunday, another federal judge, Judge Carl Nichols, ruled against Trump's order.

Some states have already begun distributing mail-in ballots ahead of the midterm elections, which will decide which party controls Congress.

It is not the first setback for Trump's postal ballots before the Supreme Court.

In June, the justices ruled that states may count ballots postmarked by election day, rejecting the Trump administration's push to block votes being tallied when they are received after polls close.

Trump has previously made false claims that he lost the 2020 election owing to widespread electoral fraud.