Theresa May is facing the prospect of a major rift at the heart of her government as pressure mounts to reform the ban on abortion in Northern Ireland.

A number of senior Conservatives called for a free vote on the issue following the Republic of Ireland’s abortion referendum, and members of the cabinet are believed to be ready to push ahead with demands for change.

The row creates a fresh headache for the prime minister because the Democratic Unionist Party (DUP), whose votes she relies on in parliament, are firmly anti-abortion and certain to resist calls for a vote on reforming the law. The cabinet is also likely to be divided on the issue.

Downing Street sources suggested Ms May would try to resist calls for a change in the law, insisting the matter should be decided by Northern Irish leaders.

But as pressure grew, one MP claimed more than 130 of her colleagues were already committed to voting for reform.

It comes as the Republic of Ireland’s comprehensive vote in favour of overturning its ban on abortion led to fresh calls for a similar move in the north of the island – the only part of the UK where the practice is still illegal.

Reports suggest Penny Mordaunt, the international development secretary, who also holds the women and equalities brief, will lead cabinet calls for reform.

Ms Mordaunt hailed the Irish vote, calling it a “historic and great day for Ireland”.

She also hinted at change within the UK, saying the result was a “hopeful one for Northern Ireland” and adding: “That hope must be met.”

The referendum in the Republic of Ireland on Friday saw 66.4 per cent of voters back repeal of the country’s abortion ban.

With Irish prime minister Leo Varadkar promising legal changes by the end of the year, it means Northern Ireland will be the only part of the UK or the island of Ireland where abortion remains illegal. The region allows a pregnancy to be terminated only if the life or mental health of the mother is at risk.

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On Sunday, Anne Milton, the education minister, called for a free vote in parliament on lifting the ban.

She told ITV’s Peston on Sunday: “It does feel anomalous ... we are offering abortions to women from Northern Ireland [in other parts of the UK]. That doesn’t feel quite right.”

“Traditionally votes on abortion have always been a matter of a free vote. I feel quite strongly about that. I do think it should be a free vote.”

She said she would vote in favour of lifting the ban, adding: “I believe in a woman’s right to choose.”

Other senior Conservatives also backed a change in the law.

Sarah Wollaston, who chairs the Commons Health Committee, said: “I would vote to support an extension of abortion rights to all women across the whole UK.

“As this is a devolved issue, if an amendment is not accepted by the speaker, then there should at very least be a referendum in Northern Ireland on this issue.”

And former education secretary Justine Greening said: “It’s clear it’s now time for debate and action to achieve the rights for Northern Irish women that we have as women across the rest of the UK.”

However, in a sign of Tory divisions on the issue, justice minister Rory Stewart warned against a vote being held in Westminster.

He told Sky News: ”It would be very, very dangerous for British politicians to be seen to be telling people in Northern Ireland how to vote.

“One of the reasons why we have a more peaceful situation in Northern Ireland is because we have delegated.”

Irish abortion referendum: The moment it was announced Ireland voted 66% in favour of repealing the eighth amendment

He added: “There isn’t a parliament in Stormont at the moment, so that puts a huge degree of trust and response on the Westminster government to have an interim government, but that mustn’t be used to make fundamental constitutional ethical changes on behalf of the people of Northern Ireland.

“It should be seen as a caretaker situation. Those kinds of decisions need to be made by the Stormont parliament when that is up and running again.”

The cabinet is also likely to be split on the issue, with a number of senior ministers, including Jeremy Hunt, Liam Fox and Chris Grayling, having previously voted in favour of lowering the time limit within which a woman can legally have an abortion.

A Downing Street source played down the prospects of the government intervening on the issue.

The source said: “The government thinks this is a matter for Northern Irish people and Northern Irish politicians, just as the Irish referendum was a matter for Irish voters and Irish politicians.

“We think it’s absolutely essential that power-sharing is restored at Stormont, and that’s where our focus is.

“The hope is that big issues like this will bring the two parties together and create an impetus and for power-sharing.”

The source also cast doubts over whether parliament currently has the power to take such a decision on behalf of Northern Ireland.

The source said: ”We’re not in the arena of direct rule yet, so there are still a few stages before the UK parliament can start legislating on behalf of Northern Ireland.

“There are a lot of hurdles it would have to clear, at this stage. The speaker’s office would have to consider it quite carefully because there are a lot of constitutional questions around it.”

Ireland votes in historic abortion referendum: in pictures Show all 15 1 /15 Ireland votes in historic abortion referendum: in pictures Ireland votes in historic abortion referendum: in pictures Members of the public hold yes placards on Fairview road Getty Images Ireland votes in historic abortion referendum: in pictures Taoiseach Leo Varadkar casts his vote at Scoil Thomas, Castlenock Dublin PA Ireland votes in historic abortion referendum: in pictures Members of the public hold no placards on Fairview road Getty Images Ireland votes in historic abortion referendum: in pictures A woman wears a campaign T-shirt and badges REUTERS Ireland votes in historic abortion referendum: in pictures A woman arrives to vote with a baby as Ireland holds a referendum on liberalizing its law on abortion, in Dublin, Ireland, May 25, 2018. REUTERS/Max Rossi MAX ROSSI REUTERS Ireland votes in historic abortion referendum: in pictures Lorraine Curtin with her son Cillian Curtin, age 5 months, and Petrina Conmee arrive at Dublin airport from Portugal, having traveled especially to vote in Ireland's referendum REUTERS Ireland votes in historic abortion referendum: in pictures Members of the public hold yes placards on Fairview road Getty Images Ireland votes in historic abortion referendum: in pictures A nun casts her ballot REUTERS Ireland votes in historic abortion referendum: in pictures Handout photo of Micheal Martin and wife Mary voting in Cork as as the country goes to the polls to vote in the referendum on the 8th Amendment of the Irish Constitution. PRESS ASSOCIATION Photo. Picture date: Friday May 25, 2018. See PA story IRISH Abortion. Photo credit should read: PA Wire NOTE TO EDITORS: This handout photo may only be used in for editorial reporting purposes for the contemporaneous illustration of events, things or the people in the image or facts mentioned in the caption. Reuse of the picture may require further permission from the copyright holder. PA PA Ireland votes in historic abortion referendum: in pictures Ireland's Prime Minister Leo Varadakar leaves a polling station after casting his vote AFP/Getty Images Ireland votes in historic abortion referendum: in pictures Tom Harrington arrives to cast his ballot at Knock national school AP Ireland votes in historic abortion referendum: in pictures President Michael D Higgins and his wife Sabina cast their votes at the polling station in St Mary's Hospial, Pheonix Park, Dublin, as the country goes to the polls to vote in the referendum on the 8th Amendment of the Irish Constitution. PRESS ASSOCIATION Photo. Picture date: Friday May 25, 2018. See PA story IRISH Abortion. Photo credit should read: Niall Carson/PA Wire Niall Carson PA Ireland votes in historic abortion referendum: in pictures Leo Varadkar casts his vote PA Ireland votes in historic abortion referendum: in pictures A woman carries her baby as she arrives to vote Reuters Ireland votes in historic abortion referendum: in pictures Nuns prepare to cast their ballots as Ireland holds a referendum on liberalizing its law on abortion, in Dublin, Ireland, May 25, 2018. REUTERS/Alex Fraser STAFF REUTERS

MPs are thought to be considering tabling an amendment to the Domestic Abuse Bill to repeal the ban in Northern Ireland.

Labour MP Stella Creasy said more than 130 parliamentarians have already expressed support for such a move.

But in a clear warning to Ms May, Arlene Foster, the DUP leader, said the issue of abortion was “a devolved mater ... for the Northern Ireland assembly to debate and decide”.

She said: “Friday’s referendum has no impact upon the law in Northern Ireland, but we obviously take note of issues impacting upon our nearest neighbour.

“A referendum was held in the Republic of Ireland because of the constitutional prohibition on abortion that existed there. No such constitutional bar exists in Northern Ireland.

“The legislation governing abortion is a devolved matter, and it is for the Northern Ireland assembly to debate and decide such issues.”

She added: “Some of those who wish to circumvent the assembly’s role may be doing so simply to avoid its decision.

“The DUP is a pro-life party and we will continue to articulate our position. It is an extremely sensitive issue and not one that should have people taking to the streets in celebration.”

Ms May is yet to offer a concrete view on the result of the Irish referendum. In a cautiously worded tweet, she said: “The Irish referendum yesterday was an impressive show of democracy which delivered a clear and unambiguous result.

“I congratulate the Irish people on their decision and all of Together4Yes on their successful campaign.”

Both Labour and the Liberal Democrats called on the prime minister to reform Northern Ireland’s abortion law.

With no imminent prospect of the Stormont government being restored, the UK government would in theory have the power to reform Northern Ireland’s abortion laws unilaterally. Senior Whitehall officials have been taking major decisions on behalf of the region since power-sharing collapsed in January 2017.

Dawn Butler, Labour’s shadow minister for women and equalities, said: “Fifty years ago, abortion was decriminalised under a Labour government, but women in Northern Ireland are still denied this fundamental right, having to travel to mainland UK or faced with potential prosecution and imprisonment at home.

“This is an injustice. No woman in the UK should be denied access to a safe, legal abortion.”

She added: “Labour’s manifesto commits to working with the Northern Ireland assembly to bring about these changes and we want to see the assembly reconvened to make such important decisions, but, nearly 18 months on, women in Northern Ireland should not have to suffer in its absence.

“We call on the government to support legislation to extend abortion rights to Northern Ireland. Labour is looking at legislative options for achieving this and urge the Conservatives to work with us to make it law.”

Sir Vince Cable, leader of the Liberal Democrats, said: “I believe in the principle of the right to choose. However, in Northern Ireland women have suffered from antiquated, inhumane criminalisation for far too long.

“The position in Northern Ireland is now highly anomalous and action will now have to be taken. Theresa May cannot remain silent on this issue.