Twitter has flagged tweets from US president Donald Trump with a fact-check warning.

The social media site added a warning phrase to two of Mr Trump’s tweets in which he called postal voting “fraudulent” and predicted that “mail boxes will be robbed”.

Under the tweets, there is now a link reading “Get the facts about mail-in ballots” that guides users to a Twitter Moments page with fact checks and news stories about Mr Trump’s unsubstantiated claims.

Until now, the US president has overcome Twitter’s half-hearted attempts to enforce rules intended to promote civility and “healthy” conversation on its most prominent user.

Mr Trump frequently amplifies misinformation, spreads abuse and uses his feed to personally attack private citizens and public figures alike – all forbidden under Twitter’s official rules.

In a statement, Twitter said Mr Trump’s vote-by-mail tweets “contain potentially misleading information about voting processes and have been labelled to provide additional context around mail-in ballots”.

Joe Scarborough was a congressman at the time. Credit: Steven Senne/AP

Meanwhile, the husband of a woman who died in Joe Scarborough’s office two decades ago has demanded Twitter remove Donald Trump’s tweets suggesting the former Republican congressman murdered her.

Twitter issued a statement expressing its regret to the husband but so far has taken no action on those tweets.

Mr Trump’s Scarborough tweets offer another example of the president using Twitter to spread misinformation – in this case, about an accidental death that Mr Trump persists in linking to the co-host of MSNBC’s Morning Joe show.

“My request is simple: Please delete these tweets,” Timothy J. Klausutis wrote to Twitter CEO Jack Dorsey last week.

The body of Lori Kaye Klausutis, 28, was found in Mr Scarborough’s Fort Walton Beach, Florida, congressional office on July 20, 2001.

Mr Trump has repeatedly tried to implicate Mr Scarborough in the death even though Mr Scarborough was in Washington, not Florida, at the time.

At Tuesday’s White House briefing, press secretary Kayleigh McEnany repeatedly refused to say why Mr Trump was pressing the unfounded allegations or whether he would stop tweeting about them.

Instead, she focused on remarks that Mr Scarborough made about the case that she said were inappropriate and flippant.