The British government brushed off a threat Thursday by its Northern Irish allies to withdraw their support if it compromises too much on Brexit, amid reports of progress in negotiations with the European Union.
Can all three ever be satisfied with the terms of a Brexit deal? Downing Street earlier warned there were "big issues still to resolve".
DUP leader Arlene Foster, who has been in Brussels this week for talks with European Union officials, repeated her warning that she would not accept anything that threatened the integrity of the UK.
Numerous problems are now being solved on a step by step basis "but there are, of course" several big issues which we really need to get to grips with, ' the prime minister said. "Such a lovely morning" is all she would say to me.
In addition, Northern Ireland would remain under large parts of single market regulations, requiring enhanced checks on products arriving from Britain, particularly agricultural goods.
Shortly after the meeting began, the DUP leader, Arlene Foster, said that May and her cabinet colleagues who had told her they were unionists "could not in good conscience recommend a deal which places a trade barrier on United Kingdom businesses moving goods from one part of the kingdom to another".
The British government and leading politicians in Northern Ireland insist they will not agree to a withdrawal treaty that foresees the possibility of Northern Ireland being kept inside the EU's economic area while the British mainland is not.
Defeat on the Budget could be a mortal blow to the PM's authority and spark a no-confidence vote in the government.
The DUP has stepped up warnings to British Prime Minister Theresa May not to compromise over the border on the island of Ireland in her efforts to secure a Brexit deal.
Theresa May has been put on notice by her allies in the Democratic Unionist Party to change course on Brexit or risk the collapse of her government.
The Irish backstop forms part of the exit treaty that the United Kingdom will sign with the European Union when it leaves; it spells out what customs and regulatory arrangements should operate in Great Britain and Northern Ireland in the event a comprehensive free trade deal can not be signed by the two parties before the end of 2020 when the Brexit transition period ends.
For customs and VAT, Barnier said, "we propose using the existing customs procedures to avoid doing checks at the transit points".