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The US Postal Service (USPS) has warned one state that its election laws might mean that millions of mail-in votes go uncounted in November's election.

In a letter to Pennsylvania's top official overseeing the vote, the USPS said that a one-week turnaround for mail-in ballots may not be possible.

It comes amid a major slowdown in mail deliveries which critics say is due to policies enacted by a Trump appointee.

A record number of people are expected to vote by mail due to the pandemic.

Mr Trump has repeatedly said that mail-in ballots will give a boost to his rival Democrat Joe Biden, and lead to voting fraud. Experts say the mail-in voting system - which is used by the American military and by Mr Trump himself - is safe from tampering.

In a letter sent late last month, the USPS warned Pennsylvania's secretary of state that mail-in ballots requested one week before the 3 November election - which the state allows under its election laws - may not reach their destination on time because the state's deadlines are too tight for its "delivery standards".

USPS General Counsel Thomas Marshall said a "mismatch" between Pennsylvania's laws and the delivery capabilities of the mail system "creates a risk that ballots requested near the deadline under state law will not be returned by mail in time to be counted under your laws as we understand them".

The letter was made public on Thursday as Secretary of State Kathy Boockvar asked the state's supreme court to allow ballots to be counted as long as they were received up to three days after the election. Currently, votes are discarded if they are received after election day.

Pennsylvania is a battleground state, which Mr Trump won by less than 1% in the 2016 election.

Neighbouring New Jersey is expected to announce on Friday that it will pre-emptively send ballots to every registered voter in the state. The process of sending out ballots is known as universal mail-in voting, and has been adopted in nine other US states.

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On Thursday, Mr Trump said he was blocking additional funding for the USPS to help with election issues because he opposed mail-in voting.

"Now they need that money in order to make the post office work so it can take all of these millions and millions of ballots," he told Fox News.

He added: "Now, if we don't make a deal, that means they don't get the money. That means they can't have universal mail-in voting, they just can't have it."

Mail wait times have climbed in the US in recent days. Critics say its due to overhauls made by the new postmaster general, who is a major Republican donor.

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