POPE Francis has taken centre stage with an attack on Donald Trump's Christian credentials while signalling a shift in the Catholic Church's stance on contraception.

In a day of unprecedented interventions, the pontiff questioned the Republican US Presidential candidate's claims to be a "proud Christian" over his attitude to deporting Mexicans and building a border wall.

Flying back to Rome from a trip to Mexico, the Pope said: “A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian.”

Addressing a rally in South Carolina, Mr Trump described the Pope's comments as "disgraceful" and claimed Francis would regret the remarks if the Vatican were attacked by so-called Islamic State (IS).

In a riposte that was extraordinary even by his standards, the Republican frontrunner said: "For a religious leader to question a person's faith is disgraceful. I am proud to be a Christian. No leader, especially a religious leader, should have the right to question another man's religion or faith.

“If and when the Vatican is attacked by Isis, which as everyone knows is Isis’s ultimate trophy, the Pope can have only wished and prayed that Donald Trump would have been president because this would not have happened."

Pope Francis, in an in-flight press conference, also indicated that women exposed to the Zika virus may be permitted to use contraception to avoid pregnancy, a departure from Catholic teaching.

He said that, unlike abortion, “avoiding pregnancy is not an absolute evil” and, in certain circumstances, it may be “the lesser evil”.

Scotland's most prominent historian, Professor Sir Tom Devine, said the vast majority of the country's Catholics were likely to be favourably disposed to the Pope's stance on contraception as a weapon against the spread of the virus, which has been linked to a wave of birth defects in Latin America.

In response to questions about whether contraception could be used to prevent Zika virus the Pope suggested artificial contraception could be used in some situations to avoid pregnancy.

The pontiff compared the situation to a decree issued by Pope Paul VI which said nuns in Africa could use birth control due to the threat of rape.

He said: "In certain cases, as in this one, such as the one I mentioned of Blessed Paul VI, it was clear.

“I would also ask doctors to do their utmost to find vaccines against these mosquitoes that carry this disease. This needs to be worked on.

"Abortion is not a lesser evil. It's a crime. It is the deliberate taking of an innocent human life. Don't confuse avoiding pregnancy with abortion."

Sir Tom, a practising Catholic, said: "This is a reflection of the man's compassion for humanity but at the same time of attempts to steer a careful path between the traditional tenets of the church and the horrible moral complexity that faces us in this world.

"I get the feeling that if talking about the Catholic community, the laity, the vast majority would certainly be understanding of what he has said on tis occasion."

Liz Leydon, editor of the Scottish Catholic Observer, said: "The Church does not believe that abortion is the answer. The sanctity of life must be protected even through such a crisis. The Church must offer pastoral care and spiritual support."

The Pope insisted his comments about Mr Trump were not designed to undermine his campaign or persuade people not to vote for him.

He said: "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 I will give him the benefit of the doubt."

During his campaign Mr Trump said he supported deporting nearly 11 million undocumented immigrants and also accused Mexico of sending its "rapists" and criminals to the US.