Pope Francis has declared that presidential candidate Donald Trump is not a Christian because of strident views on immigration and his desire to build a wall on the Mexican border. Mr Trump has hit back and called the comments “shameful”.

The Reuters news agency said that the remarkable intervention from Francis came in a in a freewheeling conversation with reporters on his flight back from a visit to Mexico. Pope Francis was asked about Mr Trump and some of his statements, such as vowing to build a wall between the United States and Mexico if he becomes president.

“A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian,” the Pontiff said in answer to a specific question about mr Trump’s views. “This is not in the gospel.”

Mr Trump has already hit back at the Pope's claims (AFP/Getty)

Asked if American Catholics should vote for someone with Mr Trump’s views, Francis said:

“I am not going to get involved in that. I say only that this man is not Christian if he has said things like that. We must see if he said things in that way and in this I give the benefit of the doubt.”

Mr Trump has worked hard to secure the support of evangelicals and conservative Christians as he has campaigned across the country. He released a campaign video in which appeared with what he said was his mother’s bible.

He also appeared at the christian Liberty University in Virginia, where he fluffed a reference to the bible but secured the support of the college’s president, Jerry Falwell, the son of the late preacher and activist Jerry Falwell Sr.

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The Republican frontrunner, who has said he would deport millions of illegal migrants if he wins the November US election, last week he told Fox Business television that Pope Francis did not understand the Mexican border issues.

“The pope is a very political person. I think he doesn't understand the problems our country has. I don’t think he understands the danger of the open border that we have with Mexico,” he said.

On Thursday, Mr Trump immediately hit back and said the pope had failed to fully appreciate the issue.

“If and when the Vatican is attacked by Isis’s, which as everyone knows is Isis’s ultimate trophy, I can promise you that the Pope would have only wished and prayed that Donald Trump would have been President because this would not have happened,” he said.