The leader of the Roman Catholic Church in Britain, Cardinal Cormac Murphy O'Connor, has plunged the church into the general election by backing a call by Michael Howard, the leader of the Conservative Party, for a reduction in the upper time limit for abortion.

The Cardinal came close to advising Roman Catholics to support the Conservatives over the abortion issue, in an echo of the campaign by the religious right in the US presidential election in support of the Republican, George Bush.

"Abortion, for Catholics, is a very key issue, we are totally opposed to it," he said last night. "The policy supported by Mr Howard is one that we would also commend, on the way to a full abandonment of abortion," said the cardinal.

"I think, as Bishops, we are not going to suggest people support one particular party," he added. "There has been a notion in the past that Catholics would be more in support of the Labour Party because they were working-class people. "Now I'm not so sure that will be quite so true today, the Labour Party has developed," the Cardinal said.

The Prime Minister has made it clear he is uncomfortable with abortion, but has said he would not support a cut in the upper time limit from 24 weeks. But Mr Howard has signalled he would support a cut to 20 weeks, and a cross-party group of MPs is launching a campaign for the law to be tightened possibly to 10 weeks, which would outlaw many abortions.

"It is very important that this debate has been opened into the public arena, both in the lead-up to and after the election," said Cardinal O'Connor.

The Society for the Protection of the Unborn Child plans to distribute information outlining MPs' views in churches and in door-to-door campaigns in selected seats, while the hardline UK Lifeleague pressure group warned that it would target a dozen pro-abortion MPs.

Interviewed by Cosmopolitan magazine at the weekend, Mr Howard made it plain he favoured reform of the law, declaring: "I believe abortion should be available to everyone, but the law should be changed. In the past I voted for a restriction to 22 weeks and I would be prepared to go down to 20."

But Mr Blair told the magazine: "Obviously, there is a time beyond which you can't have an abortion and we have no plans to change that although the debate will continue."

Charles Kennedy, the Liberal Democrat leader, said he had voted for a 22-week limit, but said medical advances meant "I don't know what I would do now".

Jim Dowson, national co-ordinator of UK Lifeleague, said the organisation would attempt to mobilise the anti-abortion vote in a series of constituencies. He said the group would distribute more than 3.5 million leaflets and was planning "high-profile stunts" in the constituencies of senior Labour figures, including Mr Blair.

Cardinal O'Connor also said the Roman Catholic Church is opposed to embryo cloning, which again will put the church on a collision course with a Labour-led Commons committee which will come out in favour of greater freedom to allow developments in fertilisation and embryology.

A leaked draft report by the Commons Select Committee on Science and Technology will reopen divisions about the rights of unborn children, when it recommends that parents should be allowed easier access to in-vitro fertilisation (IVF) and doctors should be free to screen embryos for inherited diseases.

The report also sharply criticises the Human Fertilisation and Embryology Authority for taking moral and ethical decisions, which the MPs say are not within its remit as a watchdog body.

Screening of embryos for diseases would be highly controversial and could lead to parents being allowed to choose the sex of their children.