The British government has delivered its new Brexit proposals to the EU, including plans to replace the Irish backstop.
The plan, outlined in a seven-page document, would see Northern Ireland stay in the European single market for goods, but leave the customs union - resulting in new customs checks.
The Northern Ireland Assembly would get to approve the arrangements first and vote every four years on keeping them.
The European Commission says it will "examine [the proposals] objectively".
The UK is set to leave the EU on 31 October and the government has insisted it will not negotiate a further delay beyond the Halloween deadline.
Speaking at the Conservative Party conference earlier on Wednesday, Boris Johnson said the only alternative to his Brexit plan was no-deal.
In a letter to European Commission's president, Jean-Claude Juncker, the prime minister said the new proposals "respect the decision taken by the people of the UK to leave the EU, while dealing pragmatically with that decision's consequences in Northern Ireland and in Ireland".
Government sources said they believed they could enter an intense 10-day period of negotiations with the EU almost immediately, with the aim of coming to a final agreement at an EU summit on 17 October.
John Campbell, the BBC's Northern Ireland business editor, said the UK's acknowledgement there would be new customs checks for cross-border trade would make it very hard for the Irish government to accept the package.
Northern Ireland's Democratic Unionist Party - long-term critics of the backstop and partners of the Conservative Party in Parliament - gave a cautious welcome to the proposals.
In a statement, the DUP said the plan "demonstrates commitment to working with our neighbours" in Ireland and respected "the integrity of Northern Ireland's economic and constitutional position within the United Kingdom".
But Sinn Fein said the plans were a "non-starter" and accused their former power-sharing partners of "working against the interests of the people" of Northern Ireland.
And Labour leader Jeremy Corbyn said the deal was "not acceptable" and "worse" than Theresa May's agreement, as it "undermined" the Good Friday Agreement that secured peace in Northern Ireland.
What is in the proposals?
The prime minister has set out details of his plan to replace the Irish border "backstop" in the current Brexit agreement.
The backstop is the controversial "insurance policy" that is meant to keep a free-flowing border on the island of Ireland but which critics - including the PM - fear could trap the UK in EU trading rules indefinitely.
Under Mr Johnson's proposals, which he calls a "broad landing zone" for a new deal with the EU:
- Northern Ireland would leave the EU's customs union alongside the rest of the UK, at the start of 2021
- But Northern Ireland would, with the consent of politicians in the Northern Ireland Assembly, continue to apply EU legislation relating to agricultural and other products - what he calls an "all-island regulatory zone"
- This arrangement could, in theory, continue indefinitely, but the consent of Northern Ireland's politicians would have to be sought every four years
- Customs checks on goods traded between the UK and EU would be "decentralised", with paperwork submitted electronically and only a "very small number" of physical checks
- These checks should take place away from the border itself, at business premises or at "other points in the supply chain"
The government is also promising a "New Deal for Northern Ireland", with financial commitments to help manage the changes.
What's the reaction been?
Later, Mr Johnson will speak to Mr Juncker on the phone and the two sides' negotiating teams will also meet, while the UK PM will also speak to his Irish counterpart.
German Chancellor Angela Merkel said the EU would study the proposals carefully and she "trusted" the bloc's Brexit negotiator Michel Barnier to maintain European unity.
But opponents of Brexit in Parliament indicated they would not support the proposals, unless they were accompanied by the promise of another referendum.
The Lib Dems said the proposals would deal a "hammer blow" to the Northern Irish economy while the SNP said it gave the DUP a veto over the proposed alternative to the backstop.
"This is not a way forward," the SNP's Ian Blackford told the BBC. "It is window dressing from the government."