A spokeswoman for May said changing the rules should only be undertaken by a government in Northern Ireland, which has been without a devolved executive since January last year when a power-sharing agreement collapsed.

May tweeted on Sunday to "congratulate the Irish people on their decision" but she made no mention of what the result would mean for Northern Ireland.

Northern Ireland has some of the most restrictive abortion laws in Europe with even rape and fatal foetal abnormality not considered legal grounds for a termination. Unlike in other parts of the United Kingdom, abortions are banned apart from when the life or mental health of the mother is in danger.

The penalty for undergoing or performing an unlawful abortion is life imprisonment.

Since the collapse of a power sharing administration in Northern Ireland, British officials have been taking major decisions in the region and this means the government could legislate directly despite health being a devolved issue.

But any moves to change the law could destabilise the British government by antagonising the socially conservative Democratic Unionist Party, which May depends on for her parliamentary majority.

The opposition Labour party called on the government to support legislation to extend abortion rights in Northern Ireland because women are being denied fundamental rights.

"This is an injustice. No woman in the UK should be denied access to a safe, legal abortion," said Dawn Butler, Labour's shadow minister for women and equalities.

More than 130 members of Britain's parliament, including lawmakers in the ruling Conservative party, are prepared to back an amendment to a new domestic violence bill to allow abortions in Northern Ireland, the Sunday Times newspaper reported.

Anne Milton, an education minister, on Sunday urged the Prime Minister to allow a free vote in Britain's parliament and said she thought there would be "a significant majority" in favour of liberalising the abortion laws.

Sarah Wollaston, the chair of the health select committee and a lawmaker in May's party, said she would support the proposed amendment and said Northern Ireland should at least be given a vote to decide.

This creates a fresh headache for May who is already struggling to unite her top ministers over plans to leave the European Union and is facing the prospect of a series of rebellions in parliament over her Brexit plans.

Northern Ireland's elected assembly has the right to bring its abortion laws in line with the rest of Britain, but voted against doing so in February 2016.

Reuters