Brexit talks will intensify next week as the UK said threats to tear apart Boris Johnson's deal remain on the table.
Brexit Minister Lord Frost and EU Commission Vice President Maros Sefcovic held talks in London on Friday amid ongoing disagreements over contentious trading arrangements for Northern Ireland.
Brussels offered to scrap 80% of the checks on goods travelling to Northern Ireland last month but Britain wants further alterations to the deal the PM agreed.
A UK Government spokesperson said triggering Article 16 - which allows the UK or the EU to temporarily abandon commitments made in trade agreement - was still a legitimate option.
"Lord Frost noted that there remained significant gaps to be bridged between the UK and EU positions," the spokesperson said.
"Lord Frost also underlined the need to address the full range of issues the UK had identified in the course of discussions, if a comprehensive and durable solution was to be found that supported the Belfast (Good Friday) Agreement and was in the best interests of Northern Ireland."
However the UK welcomed the "constructive spirit" of the talks and said "intensified" negotiations would take place in Brussels next week.
Mr Sefcovic said there had been a "change in tone" from Lord Frost in talks held in London on Friday.
He told a press conference: "I acknowledge and welcome the change in tone of discussion with David Frost today, and I hope this will lead to tangible results for the people in Northern Ireland."
But he said the UK needed to "reciprocate the big move the EU has made" on the Northern Ireland Protocol.
Serious progress needs to be made on Northern Ireland next week - with agreement on supply of medicines a priority, Mr Sefcovic told reporters.
He said there were areas that were "actually quite low-hanging fruit" in the negotiations.
Mr Sefcovic said: "And I believe that if our experts put their mind to it, they can actually resolve it within a week, I really believe so, because we are so close."
And he warned that "nothing's changed" regarding the EU's opposition to the UK demand to remove European Court of Justice (ECJ) oversight.
It comes as Dominic Cummings warned Tory MPs that triggering Article 16 would be a "disaster" in a lengthy blog post where he accused Boris Johnson of failing to understand the negotiations.
Mr Cummings, who was one of the architects of Brexit, tore into his former boss, who he repeatedly compared to a shopping trolley lurching from side to side. He wrote: "I do not want this government to trigger Article 16. The PM is incapable of handling it.
"If he does it’s bound to be a debacle that damages the economy and relations with allies.
"The best we can reasonably hope for given the state of No10 is to bank the EU’s concessions, muddle through for now, focus on replacing the broken trolley (PM), and return to it when he’s gone."
Earlier, Ireland's Europe Minister Thomas Byrne warned the UK against taking a "tough guy approach".
He told the Today programme: "A tough approach, or a tough guy approach, when it comes to Northern Ireland can only be counter-productive and will lead to disaster."
