The prime minister has pledged to secure a deal that would avoid a return to a hard border between Northern Ireland and the Republic of Ireland.
Theresa May, speaking in Belfast, said she understood the “real anxieties” her deal had created.
She pledged to secure a deal with the EU that “commands broad support” and a majority in Parliament.
She said her commitment to delivering a Brexit that ensures no return to a hard border was unshakable.
She said she also wanted to strengthen the UK’s “unique” relationship with the Irish government – restating the UK Government’s readiness to support the “tantalising possibility” of a joint UK-Irish bid for the football World Cup in 2030.
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She said she was “grateful” to businesses and farming groups that had supported the backstop in the withdrawal agreement.
Ultimately, it was rejected by Parliament when the deal was defeated by 230 votes.
The prime minister has now moved to ask the EU to re-open the withdrawal agreement and allow legally binding changes to be made to the backstop.
The backstop is an insurance policy designed to avoid a hard border “under all circumstances” between Northern Ireland and the Irish Republic after Brexit.
PM says she has asked Home Secretary to review cases where Irish citizens living in NI have had difficulties bringing in family members- a gesture towards nationalist concerns
— Mark Devenport (@markdevenport) February 5, 2019
End of Twitter post by @markdevenport
The UK is due to leave the EU at 23:00 GMT on Friday 29 March, when the two-year time limit on withdrawal negotiations enforced by the Article 50 process expires.
The DUP and Brexiteers believe the backstop could threaten the integrity of the union as, if it took effect, it would see checks on some goods coming into NI from Great Britain and it would also apply unless and until the EU and UK jointly decided to end it.
Mrs May said the UK government’s commitment to avoiding a hard border remained absolute.
The Republic of Ireland and EU have said there can be no renegotiation of the Brexit deal, or the backstop.
It has led to a rift in relations between the UK and the Irish government.
Mrs May said she wants to work closely with Taoiseach (Irish prime minister) Leo Varadkar and “strengthen” the bilateral relationship the UK has with the Republic of Ireland.
“There’s absolute horror among those who live in Northern Ireland and the Republic of Ireland that we could take a step backwards,” she told business leaders in Belfast.Media captionThe EU must be realistic if the Irish border issue is to be resolved, says Arlene Foster
In a gesture towards nationalist concerns, the Prime Minister said she had asked the Home Secretary to review cases with Irish citizens living in Northern Ireland have had difficulties bring in family members.
Earlier on Tuesday, DUP leader Arlene Foster said the “toxic backstop” remained the problem.
Brussels was “unfortunately turning their face against that” and needed respect for unionism in NI, she said.
Mrs Foster spoke out as the European Commission confirmed the prime minister will visit Brussels on Thursday for talks with its president Jean-Claude Juncker.
This follows a visit by Taoiseach (Irish Prime Minister) Leo Varadkar on Wednesday.
“We have heard a lot about their understanding of the Belfast Agreement, that they don’t want a hard border on the island of Ireland,” Mrs Foster told the BBC’s Good Morning Ulster programme.
“But they are quite content, apparently, to build a hard border between Northern Ireland and the rest of the United Kingdom, thereby interfering with the constitutional position of the United Kingdom.”
Where are we with the backstop?
In January, MPs overwhelmingly rejected the withdrawal deal that the government had negotiated with the EU, backing an amendment for the government to seek “alternative arrangements” to the backstop.
The prime minister addressed this and said although she has said technology could play a part in managing the Irish border, it needs to work for the “particular circumstances” of Northern Ireland.
Last July, during a speech in Belfast, Mrs May said no technology had been designed or implemented anywhere in the world yet to address the complexities of the Irish border.
European Union leaders have continued to rule out making changes to the withdrawal deal as agreed.
The EU’s chief negotiator, Michel Barnier, said there was “full agreement” that the withdrawal agreement “cannot be reopened”.Media captionConfused by Brexit jargon? Reality Check unpacks the basics
On Monday, German Chancellor Angela Merkel said the EU would listen to proposals to solve the Irish border “riddle”, although they needed to hear how the UK wanted to do it.
Tánaiste (Irish deputy prime minister) Simon Coveney said the withdrawal agreement rejected by MPs already allowed the EU and UK to work on alternative arrangements for the backstop.
“What Ireland is being asked to do by some in Westminster is to essentially do away with an agreed solution between the UK government and EU negotiators and to replace it with wishful thinking,” he said.
DUP Brexit spokesperson Sammy Wilson said his party would not support any legal “codicil” to the withdrawal deal and the prime minister would be “very foolish to go down that road”.
Sinn Féin Brexit spokesperson Chris Hazzard said he did not think the prime minister had “any credibility at all”.
“We’re dealing here with somebody here who, for the last number of months, has been trying to sell the withdrawal agreement, has been talking up the very need for the backstop when it comes to Ireland and yet, at the whim of Parliament has turned that on its head,” he said.
Analysis: A most extraordinary volte-face
By Jayne McCormack, BBC News NI Political Reporter
In November, Theresa May was in Northern Ireland as part of her tour selling her Brexit deal.
It is somewhat of a surprise that she returns to Belfast months later, seeking to get the withdrawal agreement re-opened.
It seems an extraordinary volte-face from a prime minister who is often described as stubborn and difficult.
While all the arguments about alternative arrangements to the backstop swirl around, the UK government insists it is still committed to avoiding a hard Irish border.
But the detail from Mrs May so far remains thin.
All that her speech this afternoon may succeed in doing is uniting her various critics against her.
Is the Good Friday Agreement going to the courts?
Lord Trimble, a joint recipient of the Nobel Peace Prize in 1998 for his role in the Good Friday Agreement, said he was “exploring” the possibility of a legal challenge over claims the PM’s Brexit deal breaches the historic peace agreement between the Republic of Ireland and Northern Ireland.
Mrs May’s deal “turns the Belfast Agreement on its head and does serious damage to it”, he told BBC Radio 4’s Today programme.
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He added: “Both the British and Irish governments undertook to support the agreement and what they’ve done, both of them, is broken that promise.”
Asked what the likelihood is of him launching the legal challenge was, Lord Trimble said: “If it reminds people to keep their promises, it’ll be a good thing.”
What ‘alternative arrangements’ are there to the backstop?
Alternatives to the backstop that the prime minister has said she wants to discuss with EU leaders include:
- a “trusted trader” scheme to avoid physical checks on goods flowing through the border
- “mutual recognition” of rules with the EU
- “technological” solutions
Theresa May has said she is “determined” to deliver Brexit on time, but a number of cabinet ministers have indicated they would be willing to agree to a short extension to finalise legislation for Brexit.
However, a plan to delay Brexit for up to nine months to prevent a no deal – put forward by Labour MP Yvette Cooper – was voted down by Parliament.
MPs will head into a second day of discussions on the Irish border backstop on Tuesday
What did the PM say about Stormont?
Northern Ireland has been without its executive at Stormont since January 2017.
Mrs May said the lack of a functioning assembly and executive had not helped with the Brexit deadlock, and that she is working to restore it.
“Northern Ireland does not have to rely on the Irish government or the EU to prevent borders of the past… I will not let that happen,” she added.
The prime minister will meet Stormont’s five main parties on Wednesday morning to discuss breaking the political impasse.
Mrs May also told reporters she has asked Home Secretary Sajid Javid and the Northern Ireland Secretary Karen Bradley to look at ways of securing the rights of Irish citizens living in Northern Ireland post-Brexit to bring in family members from other non-UK countries to take up residency as “a matter of urgency”.