MPs have been accused of "imposing" abortion on Northern Ireland after voting 328 to 65 in favour of relaxing the law around terminations in the province.
The vote in the House of Commons today was on an amended version of the Northern Ireland Executive Formation Bill that includes the repeal of sections 58 and 59 of the 1861 Offences Against the Person Act protecting unborn children.
The changes will come into effect if the Northern Ireland Assembly, which has been suspended since January 2017, fails to reconvene by October 21.
The legislation has drawn strong criticism from pro-lifers and conservative peers because it goes further than current UK abortion laws by permitting abortion for any reason up to 28 weeks - four weeks over the UK upper abortion limit of 24 weeks.
Michael Robinson, director of parliamentary communications at The Society for the Protection of Unborn Children, said that Theresa May's legacy would be one of undermining devolution in Northern Ireland and ushering in a "ruthless" abortion regime.
"The UK has one of the most extreme abortion regimes in the world. Since 1967, our laws have permitted the killing of more than 9 million babies and injured countless women physically and psychologically. The Abortion Act has brought only misery and destruction," he said.
"No society which is genuinely committed to equality and human rights could tolerate such a law. We are now faced with the prospect of an even more extreme situation being imposed on Northern Ireland.
"The people of Scotland, England and Wales cannot simply stand by and watch as this cruel and unjust legislation is enacted in their name."
Mr Robinson added: "Upon leaving office next week, Theresa May will only be remembered as the Prime Minister who undermined devolution in Northern Ireland and ushered in one of the most ruthless abortion regimes in the world.
"Anyone who values human life must urge the new Prime Minister to refuse to implement this inhuman and unconstitutional law."
The vote in Westminster took place despite ComRes polling revealing that a majority of people in Northern Ireland believe that abortion laws should be decided by their own lawmakers in the Northern Ireland Assembly.
Liam Gibson, SPUC's Northern Ireland political officer said that Thursday's vote in the Commons was a "disgraceful abuse of power by Westminster" that raised serious questions around constitutionality.
"It is outrageous that MPs and peers from England, Scotland and Wales cared so little for the rule of law that an overwhelming majority were prepared to disregard the right of the people of Northern Ireland to maintain legislation which has saved the lives of over 100,000 children since 1967," he said.
"The speed with which this Bill has been imposed has made a mockery of the parliamentary process. There is a complete lack of clarity regarding its provisions and the government has refused to allow time for any proper consideration of the consequences of the decriminalisation of abortion.
"By ramming abortion on demand down our throats Parliament has torn-up the devolution settlement and is treating Northern Ireland as a colony.
"There are very serious questions about the constitutionality of this legislation which the next Prime Minister will have to answer."
He added: "What is most shocking is that all this has been done in the name of human rights. Abortion is not a human right. It is an act of lethal violence directed at an unborn child and is never justified."
A letter from Baroness O'Loan urging Prime Minister Theresa May to withdraw the legislation has been signed by over 19,000 people.
Right to Life UK, a pro-life campaign group, said that the amendment represents a "very substantial" change to the law in Northern Ireland that "would leave Northern Ireland with one of the most extreme abortion laws in Europe".
"They would be far out of line with legislation in the Republic of Ireland and the rest of the UK," it said.
"If the change were to go ahead, this would potentially lead to significant numbers coming across the border for abortions from the Republic of Ireland where, post-referendum, abortion is restricted in most cases to 12 weeks gestation.
"It is also possible that there would be traffic from a number of other countries in Europe as the median gestational time limit for most abortion among EU countries is 12 weeks."
Right to Life spokesperson Clare McCarthy said that the manner in which Northern Ireland's abortion laws were being changed was "grossly disrespectful and unconstitutional".
"This horrific abortion law can be stopped. It is up to the people of Northern Ireland to ensure that Stormont reconvenes in order to protect the thousands of lives that would otherwise be lost if this extreme abortion law came into effect," she said.
"Over just a couple of days, 19,000 people from Northern Ireland signed a letter that was delivered to Theresa May yesterday, asking her to intervene and not to allow abortion to be forced on Northern Ireland.
"It is the voices of those 19,000 Northern Irish people who need to be listened to now, by their own politicians, and in their own Government in Northern Ireland.
"It's those 19,000 people, organisations and wider Northern Irish society that now need to come together as soon as possible to build a movement to ensure that Stormont is reconvened."