President Barack Obama has issued a powerful repudiation of the Republican nominee, Donald Trump, suggesting in the clearest terms yet that he does not consider him fit to be president.

He had been asked to comment on Mr Trump in the light of attacks he had launched since last week’s Democratic convention on the family of a Muslim-American soldier slain in Iraq, and foreign policy remarks at the weekend which betrayed a lack of knowledge about the circumstances of Russia’s annexation of Crimea from Ukraine.

“The Republican nominee is unfit to serve as president, I said it last week and he keeps on proving it,” Mr Obama said. “The notion that he would attack a Gold Star family, that made such extraordinary sacrifices on behalf of our country, the fact that he doesn’t appear to have basic knowledge around critical issues, in Europe, in the Middle East, in Asia, means that he is woefully unprepared to do this job."

“This is different from just having policy disagreements,” with the candidate of the other party, Mr Obama said, addressing reporters alongside the Prime Minister of Singapore, Lee Hsien Loong, who was on an official visit to the White House on Tuesday.

“There have been Republican presidents with whom I have disagreed, but I didn’t have a doubt they could function as president,” Mr Obama went on. “I think that I was right that Mitt Romney and John McCain were wrong on certain policy issues, but I never thought that they couldn’t have done the job.”

“I would have said…I know they are going to abide by certain norms and rules and common sense, they will observe basic decency, will have enough knowledge about economic policy and foreign policy…that our government will work,” Mr Obama said. "But that's not the situation here.”

Mr and Ms Khan address the Democratic convention in Philadelphia (Getty)

Mr Obama said that each time Mr Trump triggered fresh outrage on another topic, senior Republicans found themselves forced to distance themselves from their own party’s nominee. But he said the time surely should come when they also withdraw their endorsement of him, which has not happened yet.

Yet for the first time on Tuesday, a Republican member of the US Congress, Representative Richard Hanna from New York, said in an article in his local newspaper in Syracuse, that he would be voting for Hillary Clinton, the Democratic nominee, in November.

Mr Obama was meanwhile relentless in putting pressure on Republicans essentially to disavow the nominee en masse.

“The question I think they have to ask themselves is, if you are repeatedly having to say in very strong terms that what he has said is unacceptable, why are you still endorsing him? What does this say about your party that this is your standard bearer?”