The government will introduce a new law that could change post-Brexit customs plans with the EU - but No 10 denied it would "tear up" the existing treaty.
The two sides agreed in 2019 on the terms of the UK's exit, including on future trade in Northern Ireland.
Reports suggested a new law could "override" the legal force of that deal - the withdrawal agreement.
But Downing Street said it would only make "minor clarifications in extremely specific areas".
No 10 confirmed the new UK Internal Market Bill will be published on Wednesday.
The EU said the "full implementation" of the withdrawal agreement was a "prerequisite for the negotiations on the future partnership" between the bloc and the UK.
The news comes at the start of another week of negotiations on that future trade deal.
The so-called transition period - which has been in place since the UK left the EU in January - will end on 31 December and the two sides are trying to secure an agreement to take its place.
But Boris Johnson said if a deal was not reached by the European Council meeting on 15 October, both sides should "move on" - meaning the UK would go on to trade with the bloc on international trading terms.
Beyond all the talk, there is a genuine frustration in government that the EU is yet to treat the UK as if it were a fully sovereign country.
That's matched on the EU side by similar irritation that the UK won't budge.
But the bad tempers do not necessarily mean that a deal won't be reached.
And all the blood curdling vows don't mean that in the end there won't be compromise.
Labour's shadow Northern Ireland secretary, Louise Haigh, said if the government's latest moves were negotiating tactics, they were not "very effective".
She told BBC News: "It undermines all the progress that's been made over the last several months and completely jeopardises a future trading relationship."
What had the UK and EU agreed?
The two sides signed off on a withdrawal agreement last year ahead of the UK leaving the bloc on 31 January.
The document covered a number of areas, from how much the UK would have to pay for its "divorce bill" through to intentions for a future relationship.
But one of the biggest sticking points throughout negotiations had been how to handle the issue of Northern Ireland.
Both the UK and EU sides were committed to protecting the peace process in the region and preventing any reintroduction of border checks on the island of Ireland.
But they also accepted the land border between Ireland and Northern Ireland would become the UK's border with the EU, so customs rules needed to be respected and adhered to.
The UK and EU settled on the Northern Ireland Protocol.
This would see Northern Ireland continue to follow some EU customs rules after the transition period - meaning customs declarations would be needed for goods moving from Northern Ireland to Great Britain, as well as some new checks on goods going from Great Britain into Northern Ireland.
It was unpopular with some sections of the Tory backbenches and Northern Ireland's Democratic Unionist Party - which had been supporting the government until that point.
But the agreement was passed through Parliament and the Northern Ireland Protocol became part of the international treaty.
What is the UK government now proposing?
No 10 has said it is committed to the withdrawal agreement and the Northern Ireland Protocol.
And it said it was continuing its work with the EU in a joint committee to iron out issues around how it would work in practice.
However, it said it wanted to have something in place to protect trade across the four nations of the UK if an agreement was not reached by the end of the year.
The text of the bill has yet to be published, so we cannot say for definite what will be included in its wording.
But Downing Street said one thing it would do is allow ministers to unilaterally decide what particular goods were "at risk" of entering the EU when passing between Great Britain and Northern Ireland, and therefore subject to EU tariffs.
The law would also give ministers the powers to scrap export declarations on goods moving from Northern Ireland to Great Britain and would make it clear that EU state aid requirements - where governments give financial support to homegrown businesses - would only apply in Northern Ireland.
But the government insists the bill only introduces "limited and reasonable steps" to "remove ambiguity" - not "overriding" the withdrawal agreement, as government sources had suggested on Sunday.
What has been the reaction?
There has been concern from Brussels over the messages coming out from Downing Street.
European Commission President Ursula von der Leyen tweeted that she trusted the UK government "to implement the Withdrawal Agreement, an obligation under international law and prerequisite for any future partnership".
And she added that the Northern Ireland Protocol was "essential to protect peace and stability on the island and integrity of the single market".
Northern Ireland's Deputy First Minister Michelle O'Neill tweeted any threat of backtracking on the protocol would be a "treacherous betrayal which would inflict irreversible harm on the all-Ireland economy and the Good Friday Agreement".
She has co-signed a joint letter from anti-Brexit parties in Northern Ireland to the prime minister and the EU's chief negotiator, Michel Barnier, saying it would be "entirely unacceptable [to] abandon these safeguards and mitigations".
Labour accused the government of "misleading the public" over having a so-called "oven-ready deal" for Brexit.
Shadow health secretary Jon Ashworth told BBC Radio 4's Today programme: "Parliament supported the Withdrawal Agreement earlier on this year. He has made promises and signed a treaty around these arrangements for Northern Ireland, and he now seems to be backing out of that."
And Scottish First Minister Nicola Sturgeon said the move would "significantly increase" the likelihood of leaving the transition period without a trade deal, and the "resulting damage to the economy will be entirely Tory-inflicted. What charlatans".
But government sources told the BBC the legislation was "not intended to derail the talks", and a spokeswoman said the UK would continue to approach talks with the EU in good faith.
"As a responsible government, we are considering fallback options in the event this is not achieved, to ensure the communities of Northern Ireland are protected," she added.