Former French far-right leader Jean-Marie Le Pen has tweeted his support for Donald Trump's candidacy for U.S. president.

In his post on Saturday on the social media platform Le Pen, 87, said, as translated into English by The Guardian newspaper: "If I were American I would vote for Donald Trump."

"But may God protect him," Le Pen also wrote.

Polls predict that Trump, 69, a brash billionaire who has never held political office, is likely to defeat Republican party rivals to win key primary contests being held on Tuesday across America, in what is known as "Super Tuesday."

He has already unsettled mainstream Republicans by winning three straight nominating contests - in New Hampshire, South Carolina and Nevada.

Le Pen, the former head of France's National Front, built his political career calling for anti-immigration policies similar to those embraced by Trump who has promised to build a “big, beautiful wall” along the U.S.-Mexican border, and to deport 11 million undocumented people.

Le Pen once said that Nazi occupation was not "particularly inhumane" and suggested Ebola could solve Europe’s “immigration problem”. His party is now led by his daughter, Marine Le Pen, who is also known for her inflammatory rhetoric.

Trump has disavowed the endorsement of another far-right figure, David Duke, a former head of the Ku Klux Klan, who said on Friday that "voting against Donald Trump at this point is really treason to your heritage."