US President Donald Trump has called for himself and Democratic challenger Joe Biden to submit to drug tests before their first debate next month.

Mr Trump told the Washington Examiner he had noticed a sudden improvement in Mr Biden's performance in the Democratic TV debates.

The president offered no evidence his rival might be on drugs other than to say: "I'm pretty good at this stuff."

Mr Biden and Mr Trump will have three debates before the 3 November election.

Back in 2016, Mr Trump suggested his then-Democratic rival, Hillary Clinton, had been "getting pumped up" before their debates and challenged her to take a drug test ahead of their final live TV encounter. The Clinton camp brushed aside his challenge.

On Wednesday, the president - who is set to deliver his keynote address to the ongoing Republican party convention on Thursday - made a similar claim, arguing that Mr Biden's debating ability had improved markedly in the final debate.

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Mr Trump said the former US vice-president "wasn't even coherent" during some of the 11 live TV debates he competed in against a crowded field of contenders during the Democratic primary season.

By the time of the last debate on 15 March, the field had been whittled down to just Mr Biden and Vermont Senator Bernie Sanders.

Mr Trump told the Washington Examiner: "I don't know how he [Mr Biden] could have been so incompetent in his debate performances and then all of a sudden be OK against Bernie."

He added: "It wasn't that he was Winston Churchill because he wasn't, but it was a normal, boring debate.

"You know, nothing amazing happened. And we are going to call for a drug test because there's no way - you can't do that."

Mr Trump said of the debates: "Well, it is a prizefight. It's no different from the gladiators, except we have to use our brain and our mouth. And our body to stand. I want all standing - they want to sit down."

The three presidential debates will take place in Cleveland, Ohio, on 29 September; in Miami, Florida, on 15 October; and Nashville, Tennessee, on 22 October.

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Mr Trump asked for extra debates to be scheduled with Mr Biden, but the Commission on Presidential Debates declined. He then asked for the first debate to be held earlier for the benefit of early voters, again to no avail.

Both Mr Trump, 74, and Mr Biden, 77, have each traded gibes that the other is suffering from dementia.

The Biden team is yet to respond to Mr Trump's remarks.