Northern Ireland's restrictive abortion laws are in breach of human rights by failing to provide exceptions in the case of fatal foetal abnormality and sexual crime, Belfast's High Court found in a landmark ruling on Monday.

Unlike other parts of the United Kingdom, abortion is banned in Northern Ireland except only where the life or mental health of the mother is in danger. Doctors who perform abortions outside the limitations face up to life in prison.

Like in the Irish republic where the law is just as strict and debate by both sides just as fierce, the restrictions have led to thousands of pregnant women traveling across the Irish Sea to Britain for abortions each year.

After four months of deliberation, Judge Mark Horner upheld a challenge by the Northern Ireland Human Rights Commission that the laws breached the European Convention on Human Rights. He asked the parties to consider whether the ruling can be applied under current legislation.

If not, it would be referred to Northern Ireland's devolved assembly which to date has refused to extend legislation and whom the judge criticized, saying the issue was "unlikely to be grasped by the legislature in the foreseeable future".

Without citing any source, Irish national broadcaster RTE said it understood Northern Ireland's Attorney General would challenge the ruling.

Northern Ireland Human Rights Commission welcomed what they called a landmark ruling.

"Today's result is historic, and will be welcomed by many of the vulnerable women and girls who have been faced with these situations," the group's chief commissioner Les Allamby said in a statement.

The ruling is likely to have an impact on the debate south of the border where a complete ban on abortion was only lifted in 2013 when terminations were allowed if a mother's life was in danger.

With abortion law reform gaining momentum, Prime Minister Enda Kenny said last week that if re-elected he would institute a citizens' convention to debate the issue and allow his members of parliament a free vote on any recommended changes.

Kenny's junior coalition partner, Labour, with whom he wishes to return to government, has said it will campaign at elections early next year to allow abortion for cases such as rape, incest and fatal foetal abnormality.