Northern Ireland's Democratic Unionist Party (DUP), which Johnson's government relies on for support in the House of Commons, said it will be "unable to support these proposals in parliament".
(L to R): EU chief Brexit negotiator Michel Barnier, Ireland's Prime Minister Leo Varadkar, European Council President Donald Tusk and European Commission President Jean-Claude Juncker address a press conference during an European Union Summit at European Union Headquarters in Brussels on October 17, 2019.
Johnson tweeted that the two sides had struck a "great new deal" and urged United Kingdom lawmakers to ratify it in a special session being held Saturday - only the first time since 1982 that British lawmakers have been at work on that day.
The pound hit a five-month high against the USA dollar on the news.
Yet immediately complicating matters was Johnson's Northern Irish government allies, which didn't waste a minute before announcing they could not back the tentative Brexit deal because of the way it handled the Irish border.
Johnson, who was the face of the campaign to leave the European Union in Britain's 2016 referendum, has repeatedly said he will not ask for a delay - even though parliament has passed a law to oblige him to do just that if it has not agreed and ratified a deal by Saturday. Without it, it appears unlikely that parliament will sit on Saturday, as previously planned.
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The EU's top negotiator declared the backstop has been abolished. We have managed to find solutions that fully respect the integrity of the Single Market.
The UK Labour Party leader Jeremy Corbyn said that Johnson has reached "an even worse deal than Theresa May's, which was overwhelmingly rejected".
Johnson has assured his European counterparts that he can get the deal through parliament, and French President Emmanuel Macron said he was "reasonably confident" the deal could be ratified. That invisible, open border has underpinned the region's peace accord and allowed the economies of both Ireland and Northern Ireland to grow.
The conundrum was how to prevent the frontier becoming a backdoor into the EU's single market without erecting checkpoints that could undermine the 1998 Good Friday Agreement - which ended decades of conflict in the province.
According to EU's chief Brexit negotiator Michel Barnier, Northern Ireland will comply with some European Union rules on goods and will remain in the United Kingdom customs territory.
Northern Ireland would also follow the EU's rules on agrifood and industrial goods. There will be no customs checks on the island of Ireland - they will be done in ports on mainland Britain.
"As things stand, we could not support what is being suggested on customs and consent issues and there is a lack of clarity on VAT", DUP leader Arlene Foster and deputy leader Nigel Dodds said. After four years, the Northern Ireland Assembly will vote on whether to continue the arrangement or end it. "I hope it goes through this time", Finnish Prime Minister Antti Rinne said in Brussels.