Europe's top court banned on Tuesday patents of stem cells when their extraction causes the destruction of a human embryo, a ruling that could have repercussions on medical research.

The European Union Court of Justice delved into the controversial issue after a German scientist was denied a patent on a method to create nerve cells from human embryonic stem cells.

Scientists warned that the ruling would damage stem cell research in Europe, while Catholic bishops hailed it as a victory for the protection of human life.

EU law, the court said, intended to "exclude any possibility of patentability where respect for human dignity could thereby be affected".

The Luxembourg-based judges were asked by the German Federal Court of Justice to provide an interpretation of a human embryo following an appeal from the scientist, Oliver Bruestle.

Bruestle, whose patent was challenged by Greenpeace, said there were already clinical applications for his invention to treat patients with Parkinson's disease.

"This unfortunate ruling wipes out years of transnational research by European researchers in one fell swoop," said Bruestle, a University of Bonn professor.

He warned that the work by European researchers will be to the benefit of scientists abroad, who will turn it into medical products that will eventually be imported back to Europe.

The EU court said the use of human embryos "for therapeutic or diagnostic purposes which are applied to the human embryo and are useful to it is patentable".

But, the judges added, "their use for purposes of scientific research is not patentable".

"A process which involves removal of a stem cell from a human embryo at the blastocyst stage, entailing the destruction of that embryo, cannot be patented."

Blastocyst is a later stage of embryonic development, almost five days after fertilisation.

The commission of the European bishops' conference, COMECE, welcomed the ruling "as a milestone in the protection of human life in EU legislation".

The bishops said attention must be given to scientific research on alternative sources such as adult stem cells or stem cells from umbilical cord blood.

"These methods enjoy wide acceptance both on scientific and ethical grounds," their statement said.

The court wrote that "the concept of 'human embryo' must be understood in a wide sense".

An egg must be considered a human embryo as soon as a sperm enters it "if that fertilisation is such as to commence the process of development of a human being", the court said.

But the judges added that even a non-fertilised egg can be considered a human embryo when the technique used to extract an embryo can trigger "the process of development of a human being".