By Agnieszka Sawicka and Gabriela Baczynska

WARSAW, April 13 (Reuters) - Poland's parliament rejected changes to the constitution on Friday that could have toughened anti-abortion laws, dealing a blow to the ruling conservatives and their far-right allies.

Poland, which is predominantly Roman Catholic, already has one of Europe's strictest laws on abortion.

Women may undergo an abortion only when their health is threatened by pregnancy, the baby is likely to be handicapped, or the pregnancy is the result of rape.

The ruling parties proposed a range of constitutional amendments that would have either banned abortions altogether or made it harder to weaken existing anti-abortion legislation.

But in a series of votes, the coalition failed to muster a two-thirds majority needed to change the constitution. The centrist and leftist opposition voted against.

"I'm sad today as I leave parliament," Prime Minister Jaroslaw Kaczynski told reporters after the votes.

Debate on abortion heated up in Poland after a woman who nearly went blind after giving birth took her case to the European Court of Human Rights in Strasbourg.

Last month, the court ruled in the woman's favour, criticising Poland for having no way of resolving cases where doctors disagree whether an abortion is warranted.

The debate also exposed deep splits within the ruling camp, with the far-right League of Polish Families pushing for a total ban while Kaczynski's Law and Justice sought to enshrine the existing abortion law, supported by Poland's Catholic bishops. But some Law and Justice deputies, including parliamentary speaker Marek Jurek, voted for tougher regulations, splitting the party. Jurek said he would resign as the result of the vote.

Analysts said despite tensions inside the Law and Justice party and within the coalition over abortion, the government was unlikely to collapse.

"Support for all the coalition parties has fallen since (2005) elections and all three would be in a weaker position if early elections were held now," Marek Migalski, a political analyst at Silesia University, said.

Feminists and leftist parties, which say the ruling parties are putting Poland decades behind the European mainstream on social issues, applauded the result of the votes.

"It's a victory of reason over backwardness," deputy Piotr Gadzinowski of the Democratic Left Alliance told Polish television.

AlertNet news is provided by