Donald Trump's German grandfather begged the prince of Bavaria not to deport him from Germany in the early 20th century, a newly uncovered letter has revealed.

Friedrich Trump, who built up a fortune through restaurants and boarding houses after arriving in America as an immigrant, was born in the Bavarian town of Kallstadt.

But according to German newspaper Bild, he decided to return to his hometown in 1901 along with his wife, Elisabeth Christ, only to be issued with a deportation notice a few years later.

It is understood the notice was issued after the German authorities discovered he had never carried out military service before emigrating to America.