Approximately 1,000 legal abortions take place in Poland each year. The majority are carried out in cases where the unborn child has a severe and irreversible disability or a life-threatening incurable disease. Polish pro-life campaigners describe the legal provision as "eugenic."
The court's verdict will be welcomed by Catholics across Poland who prayed a novena ahead of the ruling.
The ruling will also be applauded by the hundreds of thousands of Poles who supported a citizen-initiated bill to ban abortion in cases of fetal abnormality earlier this year.
Archbishop Stanisław Gądecki, president of the Polish bishops' conference, expressed delight at the outcome.
"With this decision, it was found that the concept of 'life not worth living' is in sharp contradiction to the principle of a democratic state ruled by law. The life of every human being from conception to natural death is of equal value to God and should be equally protected by the state," he said.
He added: "While rejoicing in this epochal change of law, let us now remember that children -- who are directly affected by today's decision of the Constitutional Tribunal -- and their families should be surrounded with special kindness and real care on the part of the state, society, and the Church."
