Opposition parties are warning that a series of votes in the UK House of Commons last night has effectively set the scene for a 'no-deal- Brexit and hard border in Ireland.
The British Government narrowly avoided defeat on its Customs Bill yesterday after Prime Minister Theresa May agreed to Brexiteer demands to change its wording.
A new clause which would make it illegal for Northern Ireland to form a separate customs territory to Britain was passed without a vote.
Meanwhile, an amendment that prevents the UK from collecting taxes on behalf of the EU - unless the EU reciprocates - passed by 305 votes to 302.
The vote puts an end to Mrs May's plan for frictionless UK borders after Brexit.
Finally an amendment ensuring the UK is kept out of the EUs VAT regime was backed by 303 votes to 300.
This afternoon, Fianna Fáil's Brexit spokesperson Lisa Chambers said the amendments "effectively make a backstop agreement illegal, and could see the UK crash out of the EU next March."
“Without a backstop, there can be no withdrawal treaty or transition period, which would be utterly chaotic for all stakeholders concerned," she said.
“I sincerely hope that the Irish government has held discussions with Prime Minister May this evening to seek clarification on this latest unwelcome development.
“It is imperative that the two governments liaise with each other to ensure the best possible outcome for Ireland, the UK and the EU.”
Sinn Féin deputy leader Michelle O'Neill said the Government must now "stand firm" - noting that "We need legal certainty in terms of the back stop agreed in December."
"What Theresa May did last night was effectively tear up her own proposals from Chequers," she said.
"Her Government is in chaos and in her rush to placate the DUP and the Brexiteers, she has now made a 'no-deal' crash scenario much more likely."
Both sides in the negotiations have already agreed that there can be no return to a hard border on the island of Ireland - and the Taoiseach has insisted the Government is not preparing for such a scenario.
Hours before key votes on her government's customs bill on Monday, Mrs May had sought to avoid a parliamentary showdown with the European Research Group (ERG) of Tory eurosceptics, by accepting their amendments to the legislation.
It comes amid the ERG's fierce opposition to Mrs May's Brexit strategy, as set out in a white paper last week.
They had planned a show of strength in the Commons, but saw Mrs May bow to their demands for the UK not to collect EU tariffs unless reciprocal measures are in place; as well as committing the British government to having a separate VAT system to Brussels.
Despite claims accepting the ERG's amendments to the customs bill would compromise her Brexit plan, Mrs May earlier insisted to MPs that agreeing to the demands would "not change that Chequers agreement".
Three votes in favour
MPs subsequently voted in favour of both amendments to the customs bill, formally known as the Taxation (Cross-Border Trade) Bill, but with a narrow majority of just three votes in each case due to a sizeable rebellion by Tory Remainers.
Siding with Labour to vote against the Brexiteers' demands, 14 Conservatives sought to defeat the EU tariffs amendment, while 11 Tories also voted against the ERG's amendment on VAT.
Both amendments passed thanks to the help of a small group of Labour Brexiteers.
MPs later voted to give a third reading to the customs bill, which seeks to provide the regulatory framework for post-Brexit international trade, with the legislation now moving to the House of Lords.
Although MPs had originally been scheduled to stay in Westminster until July 24th, they could now vote on Tuesday on whether to begin their break five days early.
Such a move would bring forward the deadline for Tory MPs to force a vote of no confidence in Mrs May before they return to parliament in September.
The debate preceding Monday night's votes on the customs bill had seen heated exchanges between Tory MPs, while further criticism was made of Mrs May's Chequers plan from her party's own benches.