German archivists have found a letter written by U.S. President-elect Donald Trump's grandfather asking to be allowed to return to his German homeland after his wife failed to settle in to life in the United States.

The letter, signed by Friedrich Trump, who left Germany at the age of 16 in 1885, was unearthed at the state archives in the western German region of Rhineland-Palatinate.

A document signed by Friedrich Trump, grandfather of U.S. President-elect Donald Trump, is pictured at the state archive of Rhineland-Palatinate in Speyer, Germany November 22, 2016. RALPH ORLOWSKI / Reuters

After the turn of the century, Friedrich Trump returned from the United States to Germany, where he met and married his wife, Elisabeth, with whom he returned to New York.

"She lasted for around two years before making it known that she wanted to return to Germany," Franz Maier of the Rhineland-Palatinate state archives in Speyer told Reuters Television.

Friedrich had, however, failed to deregister properly before he left Germany and was not therefore allowed to be renaturalised in Bavaria, which was then its own kingdom.

An accompanying document in the archived dossier read: "The settlement in Bavaria cannot be permitted."

That fateful rejection set off the series of events that ended up with Donald Trump being elected U.S. president.