In a strongly worded sermon to be delivered this Thursday afternoon at St Mary's Cathedral in Edinburgh, Cardinal Keith O'Brien will say that the claims and assurances made at the time of the passing of the 1967 Abortion Act were no more than a "pack of lies".

He will add that the assurances amounted to "lies and misinformation masquerading as compassion and truth".

Cardinal O'Brien's words come as the 40th anniversary of the act approaches and are timed to coincide with the Catholic Church's "Day for Life" on 31 May, running this year under the theme "Blessed is the fruit of your womb".

The Scottish Day for Life falls on The Feast of the Visitation which marks the journey by Mary who went to visit her cousin Elizabeth at the same time she was expecting a child. At their meeting, John the Baptist, the child in Elizabeth's womb leapt for joy, and Elizabeth cried out: "Blessed are you among women and blessed is the fruit of your womb."

The day will be marked in Scotland's 500 Catholic parishes who have been sent 250,000 leaflets expanding on the theme and explaining the church's opposition to abortion and providing details of alternatives.

In a letter which accompanies the material sent to parishes, Archbishop Mario Conti urged every parish in Scotland to "take the opportunity to remind people that it is 40 years since the Abortion Act was passed into law". He added that people should be encouraged to "pray for legislation to protect the unborn child from the moment of conception".

In his sermon on Thursday, Cardinal O'Brien will also call on Catholic politicians to avoid "cooperating in the unspeakable crime of abortion", reminding them of "the barrier such cooperation erects to receiving Holy Communion".