Edinburgh, Scotland - Catholic politicians who defend abortion should not expect to remain full church members, Scotland's most senior Roman Catholic has warned.
In a sermon marking 40 years since the Abortion Act, Cardinal Keith O'Brien said pro-abortion MPs should consider their stance on receiving Communion.
At Edinburgh's St Mary's Cathedral, he has urged voters to reject candidates who defend a "social evil".
The cardinal's opponents have accused him of using inflammatory language.
They said it was up to elected officials to decide such ethical issues without facing threats from church leaders.
Abortion remains a reserved issue, legislated over by politicians at Westminster.
BBC religious affairs correspondent, Robert Pigott, said Cardinal O'Brien has been frustrated by what he sees as the marginalisation of Christian values in public affairs.
He has embarked on his boldest intervention yet in political life, our correspondent said.
His sermon attacked the 1967 act, describing the roughly seven million abortions in Britain since then as an "unspeakable crime" and the "wanton killing of innocents".
The cardinal told the BBC: "We're told by statisticians that the equivalent of a classroom of children every day are being aborted in their mothers' womb - basically murdered in their mothers' womb."
However, his intervention has angered some politicians. Jeremy Purvis, a Liberal Democrat member of the Scottish Parliament, said the cardinal was using "inflammatory" language.
He said: "It is not right that we would be seen to be put under pressure, or indeed some members threatened, by a religious leader on what is a very sensitive issue."
A Liberal Democrat spokesman in Westminster said abortion was a matter for the individual conscience and that Mr Purvis was giving his own personal view as a liberal politician.
That view was echoed by the main parties in Scotland.
Minister for public health, Shona Robison, said that the Scottish government was committed to improving Scotland's sexual health and reducing the number of unintended pregnancies through Respect and Responsibility, the national sexual health strategy.
'Derelicting its responsibility'
SNP MSP Michael Matheson said the Catholic Church would be "hypocritical" if it didn't speak out against abortion and those who supported it.
He added: "I think it is perfectly reasonable for the Catholic leader in Scotland to effectively assert Catholic social teaching that is opposed to abortion.
"If you had a situation where politicians who were voting in favour of abortion were then expected to be able to participate fully within a Catholic Mass then the church itself would be derelicting its responsibility to uphold its own teachings."
Harry Conroy, editor of the Scottish Catholic Observer, said the paper fully supported Cardinal O'Brien's stance on abortion.
Mr Conroy denied that the cardinal had used extreme language.
"What Cardinal O'Brien has done is bring it up in a manner that makes people stop and listen," he said.
Ann Furedi, chief executive of the British Pregnancy Advisory Service, said: "Abortion is a safe, legal and medically legitimate procedure, essential in safeguarding women's reproductive health.
"Women should be able to make their own choices about their bodies and their future. The cardinal has every right to hold his views. He obviously doesn't have to have an abortion, or provide one."