Poland's top court has ruled a law allowing abortion of foetuses with congenital defects is unconstitutional.

The decision by the country's Constitutional Court on Thursday effectively bans terminating pregnancies in cases when birth defects are found and will further limit access to abortions in Poland.

Two judges in the 13-member court did not back the majority ruling.

The ruling came in response to a motion from right-wing politicians who argued that terminating a pregnancy due to such defects violated the provision of the Polish Constitution protecting the life of every individual.

The challenged law was introduced in 1993 as a hard-won compromise that also allows abortions when a pregnancy endangers a woman's health or life, or results from rape or other illegal act.

Explaining its decision, the court said there can be no protection of the dignity of an individual without the protection of life.

The verdict was announced by the court's president, Julia Przylebska, a loyalist of the right-wing government.

Abortion rights groups had held protests and international human rights organisations argued against further restricting abortions.

In response to the decision, Council of Europe Commissioner for Human Rights Dunja Mijatovic tweeted that it is a "sad day for women's rights".

"Removing the basis for almost all legal abortions in #Poland amounts to a ban & violates #HumanRights," she said.

"Today's ruling of the Constitutional Court means underground/abroad abortions for those who can afford & even greater ordeal for all others.

"A sad day for #WomensRights."

Polish politicians considered legislation earlier this year that would have imposed a near-total ban on abortion but ended up postponing a final vote on the proposal brought by a Catholic group.

Similar efforts have been mothballed in the past after mass protests (pictured 2016).