WARSAW (Reuters) - Poland's main opposition party called on Thursday for changes in the law to allow pregnancies to be terminated on demand, in a substantial policy shift amid growing strife over abortion in the predominantly Catholic nation. However while the centrist Civic Platform (PO) announced a change in its platform, legislative changes are unlikely in the current parliamentary term, which is due to continue until 2023. A Constitutional Court ruling mandating a near total ban on abortion from last October has upturned nearly three decades of broad consensus in Poland that abortion should be allowed only in the case of rape, incest, a threat to the mother's health and foetal abnormality. The ruling also exposed growing support among young voters in particular for a liberalisation of abortion rules in line with the European mainstream, despite the nationalist government's backing of the court verdict. The PO said on Thursday it wanted women to have access to abortions at up to 12 weeks of pregnancy in difficult situations after consulting with a doctor and psychologist, while also calling for broader access to sex education, birth control, in vitro and prenatal testing. This is a response to what our voters expect. A clear stance on this matter, PO head Borys Budka told a news conference. Political observers say young voters, many of whom filled the streets with protests for weeks after the court ruling, may be crucial to the outcome of the next parliamentary election, due in 2023. Opinion surveys have shown a sharp turn towards the left among youth, while the ruling Law and Justice (PiS) appeals to older, poorer voters. Budka's PO has relied on centrist voters and moderate conservatives for over two decades. A February poll published by Polish daily Dziennik Gazeta Prawna showed that over 40% of Poles, especially younger voters, believe abortion rules should be liberalised. An SW Research poll conducted soon after the court ruling indicates that over 70% of Poles were against the decision to further restrict abortion rights in the country. (Reporting by Joanna Plucinska; Editing by Justyna Pawlak and Frances Kerry)