Anti-brexit protestors wave flags outside the Houses of Parliament in London, Britain, October 11, 2018.
Chancellor Philip Hammond became the first senior Government figure to suggest that the backstop - created to keep the border between Northern Ireland and the Republic open in the case that no broader EU/UK trade deal is finalised - will "probably" have to come into effect for a period.
Prime Minister Theresa May relies on her DUP allies' 10 MPs to prop up her minority Government in key votes and there have been suggestions the Northern Irish party could vote against the Budget.
It is understood that a number of ministers voiced concern at a meeting with the PM in Downing Street on Thursday that any such backstop could leave the United Kingdom in an open-ended customs union, preventing it from seeking trade deals elsewhere in the world.
Asked about the threat, May said: "The DUP will do what the DUP will do". That will become the only land border between the United Kingdom and the bloc.
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But Eurostar spokesman Rob Haycocks said the company's trains, which stop at Ashford International and Ebbsfleet International, should follow existing services come March: "We are having constructive conversations with the governments on Brexit and will continue to do so". If she can not convince them that she is able to deliver a Brexit deal that her European counterparts see as respecting the EU's joint rules on trade and investment, negotiations are expected to fall into crisis.
"We are not there yet".
Still, diplomatic sources on both sides of the talks told Reuters that "it was going well" and that Brexit negotiators locked away in Brussels this week were "making headway" specifically on the Irish issue.
"Anyone engaging in this in a light-hearted way foolishly fails to grasp the gravity of the decisions we will make in the coming weeks".
The bloc's national leaders meet for a high-stakes summit on Wednesday, hoping to declare "decisive progress" in the divorce talks and announce another extra Brexit summit to finalize the deal, including an offer of close future ties with Britain. Theresa May and the European Union leaders are expected to reach a final agreement after negotiations in Brussels on October 16.
"But it is true that there needs to be a period, probably following the transition period that we have negotiated, before we enter into our long-term partnership, just because of the time it will take to implement the systems required", he said.
By withdrawing its support, the DUP could make it hard for May to pass legislation through parliament, including the budget which will be voted on later this month.
The parliamentary arithmetic is hard for May who commands a majority of only 13 lawmakers including the DUP.
There is no hard border at present, as goods and services are traded between the two countries with few restrictions as both are now part of the EU single market and customs union.
As many as three Cabinet members could resign because of the backstop - Esther McVey and Penny Mordaunt as well as Leadsom - and speculation that disaffected Tories could force their party into another leadership battle, raising the prospect of another general election a no-deal Brexit, and even a second referendum.