A commitment that there will be no "physical infrastructure, including border posts, or checks and controls" in Northern Ireland has effectively killed a no deal scenario with the EU, MPs have heard.
Brexit Secretary David Davis' decision to include the provision in the EU (Withdrawal) Bill has also made the maximum facilitation or "max fac" option -- which would see new technology used to deal with cross-border trade -- "unlawful".
Tory Remainers and Labour's shadow Brexit secretary Sir Keir Starmer, debating the Brexit bill for a second day in the Commons, said the inclusion of the Northern Ireland amendment meant the only option was to now "reproduce the customs union and the single market".
Sir Keir said: "If maximum facilitation does involve infrastructure checks or controls, it would be unlawful under the provision passed yesterday, therefore it cannot happen.
"The only answer to no hard border in Northern Ireland in the end is a customs union and high levels of market alignment, the fact that was accepted by the Government and turned into domestic law gives it a status it didn't have until yesterday."
Tory MP Heidi Allen (South Cambridgeshire) responded to his point, saying: "It's huge, it actually says I think logically we will have to come to a customs union agreement, partnership, love dance, don't care what you call it, that's what we will need to avoid any border to Northern Ireland."
Former attorney general Dominic Grieve also agreed, saying: "Not only will we have to stay in a form of customs arrangement amounting to a union, but we're also going to have to have a high level of regulatory alignment because otherwise the life that takes place along the border will be impossible because of different regulations on either side."
Tory grandee Ken Clarke said: "It was the most significant thing that happened yesterday, but in the cirtus that surrounded everything and the timetable stopped us debating it nobody.
"The legally binding commitment yesterday extends the needs of the Irish border to the whole of the United Kingdom.
"So we're talking about Dover and we're not having a border down the Irish Sea so the United Kingdom has got to negotiate an arrangement with the EU as a whole, which has no new frontier barriers.
"So effectively we are going to reproduce the customs union and the single market and the Government will not be able to comply with Tuesday's legal obligation unless it does so."
Solicitor General Robert Buckland later said leaving with a no deal and trading under World Trade Organisation rules would be inconsistent with Government policy on the Northern Ireland border.
It followed a question from Labour's Chuka Umunna (Streatham), who asked: "Does he accept that if we were to leave with no deal and we were trading on WTO rules, that under WTO rules would necessitate a border, therefore leaving with no deal is inconsistent with Government policy as he has just stated it?"
Mr Buckland replied: "I entirely agree: the Government's policy is to achieve a deal because we are mindful of the points that he and others understand."