Donald Trump, the US president, has been accused of "shocking ignorance" after asserting that Korea "used to be a part of China" shortly after a meeting with Xi Jinping, the Chinese president.

Mr Trump made the comment in an interview with The Wall Street Journal on April 12, although the quote was only picked up on subsequently by the online news website Quartz.

In the interview, Mr Trump said of his Chinese counterpart, "Went into the history of China and Korea... And you know, you're talking about thousands of years ... and many wars. And Korea actually used to be a part of China".

The interview with the Journal was the same one in which Mr Trump revealed that the Chinese leader had explained the situation on the Korean Peninsula and he concluded, "After listening for 10 minutes, I realised it's not so easy".

"I suspect that Mr Xi said, in effect, that Korea was part of China because it was overwhelmingly under Chinese influence historically and Mr Trump bought that," said Rah Jong-yil, a former South Korean ambassador to both London and Tokyo.

"It shows his shocking ignorance of the situation in north-east Asia," he told The Telegraph. "That is very disturbing to us".

There is a growing body of nationalist thought in China that ancient Korean kingdoms were part of the Chinese empire and that modern-day nation states should similarly fall within Beijing's exclusive sphere of influence, Mr Rah said.

"It is true that the Korean peninsula was under the influence of China, but that was under the Ming dynasty - which was a long time ago and nothing to do with the People's Republic of China", he said.

"In the distant past, Korea may have looked up to China as a model of political or economic development, but today we consider the communist-led nation to be economically, politically and socially backward."

The bigger problem, he suggested, is that the leader of South Korea's most important security ally is being schooled in the history of the region by a neighbouring power hostile towards South Korea.

"Somebody needs to enlighten Mr Trump about the facts of the region and he should not fall for this sort of silly nationalism from the Chinese," Mr Rah added