The Government has been forced to deny that a measure it quietly adopted on the Irish border this week effectively kills off the chances of a 'no deal' Brexit and locks the UK into a tight customs arrangement with the EU.
Ministers were also quick to deny that the amendment rules out the possibility of the UK adopting the customs arrangements preferred by Brexiteers.
But both Tory and Labour MPs now say the little debated measure, making illegal any "physical infrastructure, including border posts, or checks and controls", is the most significant thing to have occurred in this week's fraught Commons Brexit debates.
It comes amid a growing concern among Tory Brexiteers that the Government is slowly moving towards a deal involving something looking very similar to the existing EU customs union.
It has simply accepted a plan passed by the Lords that enshrines in law the commitment to have no infrastructure at the Irish border.
With excitement focused on the row over parliament's role in Brexit, it was only yesterday that MPs started to point out the importance of the Irish border amendment.
Ex Tory-chancellor Ken Clarke said: "It was the most significant thing that happened. The legally-binding commitment extends the needs of the Irish border to the whole of the UK.
"So we're talking about Dover, and we're not having a border down the Irish Sea, so the UK has got to negotiate an arrangement with the EU as a whole, which has no new frontier barriers.
"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."
Tory MP Heidi Allen agreed, saying that "logically we will have to come to a customs union agreement ... to avoid any border to Northern Ireland".
Tory former Attorney General Dominic Grieve added: "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."
A source from Brexit Secretary David Davis' department denied the amendment itself committed the UK to a kind of customs union, pointing out that the Government has said it will keep the border open, even if it means keeping it free of infrastructure on the UK side.