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 15 October, both sides should “move on” – meaning the UK would go on to trade with the bloc on international trading terms.
Labour’s shadow Northern Ireland secretary, Louise Haigh, said if the moves were a negotiating tactics, they were not “very effective”, adding: “It undermines all the progress that’s been made over the last several months and completely jeopardises a future trading relationship”.
The future of Northern Ireland was a major sticking point during Brexit negotiations on the withdrawal agreement.
Both sides were committed to protecting the peace process – preventing any reintroduction of border checks on the island of Ireland – but also ensuring customs rules were respected and adhered to.
The UK and EU settled on the Northern Ireland Protocol, which would see the area 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.
But government sources say the new law could remove the need for these checks, contradicting the protocol.
Downing Street denied the claim, saying it was still committed to implementing the withdrawal agreement and the Northern Irish Protocol.
A government spokesman said: “We are taking limited and reasonable steps to clarify specific elements of the Northern Ireland Protocol in domestic law to remove any ambiguity and to ensure the government is always able to deliver on its commitments to the people of Northern Ireland”.