The government has been forced to deny that a measure it quietly adopted 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 – passed without a vote – rules out the possibility of the UK adopting the future 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 pro-Brexit Conservative MPs that the government is slowly moving towards a deal involving something looking very similar to the existing EU customs union.

The Independent was among the few organisations to cover the move by the government to simply accept a plan passed by the Lords that enshrines in law the commitment to have no infrastructure at the Irish border.

With excitement focussing on the row over parliament’s role in Brexit, it was only on Wednesday 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 yesterday.”

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He went on: “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.

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 Conservative MP Heidi Allen

“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.”

Tory MP Heidi Allen agreed, 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 Tory attorney general Dominic Grieve, who has led much of the technical debate on the government’s approach to Brexit, 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.”

Nicky Morgan: Parliament will have a 'real say' if the government fails to secure a Brexit deal

The government is still trying to formulate what kind of customs arrangements to seek in EU negotiations but the main option preferred by Brexiteers and Brexit Secretary David Davis is the so-called maximum facilitation or “max fac” option.

It would see new technology used to deal with cross-border trade, reducing the need for border checks.

But Labour’s shadow Brexit secretary Sir Keir Starmer 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.”

Brexit so far: in pictures Show all 53 1 /53 Brexit so far: in pictures Brexit so far: in pictures Brexit campaign Boris Johnson led the VoteLeave campaign PA Brexit so far: in pictures Brexit campaign Boris Johnson MP, Labour MP Gisela Stuart and UKIP MP Douglas Carswell address the people of Stafford in Market Square during the Vote Leave Brexit Battle Bus tour on 17 May 2016. Their lead line on the tour was: “We send the EU £350 million a week, let's fund our NHS instead.” Getty Brexit so far: in pictures Voting day A man shelters from the rain as he arrives at a polling station in London on 23 June 2016. 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May and Macron agreed a new border security deal, through which the UK will pay more to France to stop migrants trying to reach British shores on 18 January 2018 AFP/Getty Brexit so far: in pictures Transition period agreed The UK and EU agree terms for Brexit transition period on 19 March, 2018 Reuters Brexit so far: in pictures No agreement on Irish border The EU and UK however failed to reach an agreement on the Irish border during the successful talks on other Brexit issues AFP/Getty Brexit so far: in pictures EU attacks May’s ‘fantasy’ strategy For months after the March deal is struck there is little significant progress in talks. 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Getty Brexit so far: in pictures People's Vote march As the People's Vote campaign and The Independent's Final Say campaign gain traction, 700,000 people turn out in London to demand a final say on the UK's Brexit deal on October 20 2018 PA Brexit so far: in pictures More resignations As the Prime Minister settles on a Brexit deal, Brexit secretary Dominic Raab resigns along with Work and Pensions secretary Esther McVey and many other ministers Getty Brexit so far: in pictures Final Say petitions delivered to Downing Street People's Vote supporting MPs Chukka Umunna, Justine Greening and Caroline Lucas and The Independent editor Christian Broughton deliver over a million signatures in favour of a People's Vote to the Prime Minister at 10 Downing Street on December 3 2018 PA Brexit so far: in pictures May delays vote On December 10, the Prime Minister delayed the vote on her Brexit deal as it was near certain not to pass through the Commons due to Tory rebels and lack of DUP support AFP/Getty Brexit so far: in pictures No confidence motion Tory MPs triggered a confidence vote in the Prime Minister on December 12. 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The prime minister did not appear in parliament to see another defeat PA Brexit so far: in pictures Labour and Conservative MPs resign and create the Independent Group Back row of Chris Leslie, Gavin Shuker, Chuka Umunna and Mike Gapes, middle row of Angela Smith, Luciana Berger and Ann Coffey and front row of Sarah Wollaston, Heidi Allen, Anna Soubry and Joan Ryan PA Brexit so far: in pictures Non-biding votes on amendments to Brexit motion On February 27 he house held a series of votes, unanimously calling for the UK and EU to guarantee citizens rights in a no-deal scenario AFP/Getty Brexit so far: in pictures Attorney General publishes legal advice A hammer blow for May as Geoffrey Cox said her renegotiated deal can still leave UK in backstop against its will. Mr Cox did say the prime minister’s efforts had reduced the risk of the UK being trapped in the backstop indefinitely. MPs went on to vote against her deal by 391 to 242 UK Parliament/PA Brexit so far: in pictures No-deal off the table MPs rejected a no-deal Brexit by 43 votes on March 13, with cabinet ministers rebelling in another humiliating defeat for Theresa May. A day later they voted in favour of the prime minister seeking an extension to Article 50 AFP/Getty Brexit so far: in pictures House speaker bans May from third Commons vote on same Brexit deal John Bercow sensationally told Theresa May he would stop her making another attempt to pass her Brexit deal unless she has secured changes. The Speaker said a further ‘meaningful vote’ would be ruled out of order if the motion was “the same or substantially the same’ – under an ancient convention to stop the government bullying parliament on issues MPs have rejected Parliament Live Brexit so far: in pictures May writes to Tusk The prime minister wrote to Donald Tusk, president of the European Council, to ask for a three-month extension to give her more time to try to get her deal through parliament. However the European Commission advises the EU27 should offer a short extension to May 23 or a longer one meaning the UK would participate in European elections 10 Downing Street/AFP/Getty Brexit so far: in pictures European Council summit Theresa May’s request to extend triggering Article 50 until the end of June was rejected by the EU, and instead offered a shorter time frame. She accepted the offer of a delay until May 22 if her withdrawal deal is approved by Parliament. If MPs rejected it for a third time, the EU said Britain must propose a new plan by April 12. Ms May said she will not support a long delay because it would mean Britain participating in elections for the European Parliament Getty Brexit so far: in pictures Brussels confirms preparations for a no-deal Brexit are completed They warned that it is “increasingly likely” the UK will crash out. In a statement the European Commission (EC) said “preparedness and contingency work”, which the EC has been conducting since December 2017, was now finished. The announcement came days after EU leaders agreed to a request by Theresa May to extend the UK’s Brexit date AFP Brexit so far: in pictures May resigns British Prime Minister Theresa May reacts as she delivers a statement in London, Britain, May 24, 2019. REUTERS/Simon Dawson SIMON DAWSON Reuters

When Labour MP Chuka Umunna challenged Conservative minister Robert Buckland over whether the amendment now meant a “no deal” Brexit is “inconsistent with Government policy”, he said “I entirely agree”.

The comment seemed to fly in the face of Theresa May’s promise that “no deal, is better than a bad deal”, but a source from Mr Davis’s Department for Leaving the European Union, said it did not because Ms May has always also said that she is not aiming for a “no deal” situation.

The source also denied that 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.

They also argued that the government’s proposal for a maximum facilitation customs arrangement, would not mean infrastructure at the border itself but elsewhere, and so it also would not be ruled out by the amendment.

An official DExEU spokesperson said: “We have made clear our commitment to avoiding a hard border between Northern Ireland and Ireland, as well as avoiding any borders within our United Kingdom.