Abortion has become the latest clear dividing line between the Labour and SNP candidates in the Glasgow East by-election after the Catholic Church turned up the heat on MPs over the UK Government's plans for more embryo research.

In the contest's first televised head-to-head debate, Labour's Margaret Curran, accepting the sensitive nature of the abortion issue, said she would back the UK Government line of keeping the time limit to 24 weeks.

However, she stressed how she understood the passions involved on both sides of the embryology argument and politicians were "obliged to hear those arguments", adding that she had "great respect" for people from faith communities.

In contrast, John Mason, the Nationalist candidate, came out firmly to lower the abortion time limit. Stressing how he was "from a faith-community background", he declared: "I'm extremely unhappy about experiments with babies or research or anything like that."

He made clear the issues in the Human Fertilisation and Embryology Bill were ones of conscience and should not be whipped, as the UK Government was doing, but added that there were other possible ways to do medical research.

Ian Robertson, for the Liberal Democrats, while also recognising the sensitive nature of the debate, said he would back the UK Government line on embryo research as it was important work while Davena Rankin, the Conservative candidate, said there was no clear medical evidence at present to reduce the abortion time limit.

"What we need to have is a sensible and balanced debate. There are too many people in this debate using emotive terms as John Mason did earlier," she added.

Today would have been the key Commons debate on the Embry-ology Bill but the UK Government unexpectedly announced it was being shelved until the autumn, ostensibly to allow MPs more time to consider the issues. However, it was claimed by opposition parties that ministers were fearful of a Catholic backlash in Glasgow East should the issue of abortion and embryo research hit the headlines in the run-up to the July 24 poll, an assertion denied by Labour. As many as one-third of the constituency's voters are Catholic.

Last night, the Catholic Church continued to pile pressure on MPs, urging them to look to their consciences and not back the bill.

After Bishop Joseph Devine, whose diocese covers Glasgow East, launched a devastating onslaught on Gordon Brown's government over the bill, accusing it of "violating moral law" and "losing ethical credibility", Cardinal Keith O'Brien, Scotland's leading Catholic churchman, again targeted MPs.

Noting how they would now have the summer to contemplate the bill, he again described it as a "monstrous attack on human rights, human dignity and human life".

Speaking while on a pilgrimage to Lourdes, Cardinal O'Brien referred to the 200,000 abortions in Britain each year and noted: "By the time I retire in five years, approximately another one million babies could have been killed in the womb in our country."

He added: "I now call on all of our MPs to search their hearts and their consciences in this extra time in which they have been given and to decide whether or not the value of human life really matters or whether or not it is simply one more commodity to be cast aside in our throwaway society."

Labour and the SNP each hailed the first poll in Glasgow East as good news. The ICM snapshot put Labour on 47% and the Nationalists on 33%. Compared with the 2005 General Election result, this is a drop of more than 12% for Labour and a rise of 16% for the SNP.

Ms Curran said the poll was "progress" for Labour but stressed that her party was taking nothing for granted. Mr Mason noted how, given the SNP needed a 22% swing to win the seat, it was two-thirds of the way there.