A post by US President Donald Trump has been given a fact-check label by Twitter for the first time.
President Trump tweeted: "There is NO WAY (ZERO!) that Mail-In Ballots will be anything less than substantially fraudulent."
Twitter put a warning label under the post and a subsequent tweet under its new policy on misleading information.
Mr Trump responded by tweeting again, saying the social media giant "is completely stifling free speech".
Twitter's notification displays a blue exclamation mark underneath the tweets, suggesting readers "get the facts about mail-in ballots".
What are mail-in ballots?
They are voting bulletins that are distributed and returned by post.
In a recent opinion survey conducted by the Pew Research Center, 66% of Americans said they would not be comfortable going to a polling place to cast their ballot during the current coronavirus outbreak.
Such concerns have increased pressure on states to expand the availability of mail-in ballots for all voters in order to minimise the risk of viral exposure from in-person voting.
While every state provides some form of remote voting, the requirements to qualify vary greatly.
Five states in the western US, including Washington, Oregon and Colorado, conduct their elections entirely via mail-in ballot.
Others, like California, provide a postal ballot to anyone who requests it.
On the other end of the spectrum, 17 states require voters to provide a valid reason why they are unable to vote in-person in order to qualify for an absentee ballot.