Pope Francis has slammed Republican presidential candidate Donald Trump’s views on US immigration as “not Christian”, prompting the billionaire businessman to launch an attack of his own, labelling the religious leader “disgraceful” for questioning his faith.

Trump, the longtime party front-runner in national opinion polls, has been at the centre of controversies, including one over his vow as president to build a wall between the United States and Mexico to keep out illegal immigrants.

In a freewheeling conversation on his flight home from a visit to Mexico, Francis told reporters “A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian.”

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Trump, a real estate developer and former reality TV star, said “If and when the Vatican is attacked by the ISIS, 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,” Trump said in a speech in South Carolina, using an acronym for the Islamic State militant group.

Pope Francis questioned Trump’s immigration policies. Photo: ABC

“For a religious leader to question a person’s faith is disgraceful. I am proud to be a Christian and as president I will not allow Christianity to be consistently attacked and weakened,” Trump said in a news release responding to the Pope.

“No leader, especially a religious leader, should have the right to question another man’s religion or faith,” Trump said.

Trump has also said he would deport millions of illegal immigrants if he wins his party’s nomination and then the November 8 election. Last week, responding to the Pope’s plan to visit the US-Mexican border, he said 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.

Asked about being called a “political person”, Francis said on Thursday “Thank God he said I was a politician because Aristotle defined the human person as ‘animal politicus.’ So at least I am a human person.”