Northern Ireland must comply apply European Union customs codes as well as standards in goods, EU advisor Stefaan De Rynck said on Wednesday.
De Rynck made the comments at a speech at the London School of Economics, which aimed to discuss concerns over post-Brexit trade and finance.
The chief advisor to EU chief Brexit negotiator, Michel Barnier, also said that trade talks would be tougher than negotiations that secured the UK's divorce settlement, due to the broader scope of issues and deadline.
EU officials had told the United Kingdom 'well in advance' that free trade deals would require London to uphold comittments to fair competition, he added.
EU Approach to London-Brussels Negotiatioins
The EU "respects" that the UK government has said that Brexit meant "friction and the creation of non-tariff barriers to trade", but added that Mr Barnier had said in prior comments that "if you leave the single market, there will be friction", De Rynck said as reported by Sky News political correspondent Rob Powell.
— Rob Powell (@robpowellnews) February 19, 2020
The EU "precondition" of a robust level playing field guarantee was "no surprise" as it had been addressed by the EU27 as early as March 2018, De Rynck said.
Every free trade agreement was tailored to the third country involved, he added, responding to comments from Downing Street tweeted on late Tuesday.
— No.10 Press Office (@Number10press) February 18, 2020
The UK's Brexit plans were a "source of concern" and that its "level of ambition" was not being met as stated in last year's agreement, he added, citing a ministerial statement released earlier in February.
De Rynck added that the closest possible security relationship would require work with the European Court of Justice if EU concepts were to be used, including adherence to the European Court of Human Rights.
The UK's decision to end freedom of movement in the country was no surprise, he said, adding that “free movement has been a tremendous benefit for this country economically and culturally”.
— Stefaan De Rynck (@StefaanDeRynck) February 18, 2020
EU, UK Prepare For 'Long Road Ahead'
The news comes after Barnier said that the EU would "never, never, never" compromise on the integrity of the common market, adding that London would face the reality after 'underestimating' the costs of leaving the EU.
The top EU Brexit chief made the comments at the Queen's University in Belfast amid uncertainty over the Irish backstop, which would enforce physical custom checks at the border between the country and Republic of Ireland, Reuters reported in late January.
He said at the time: “Leaving the single market, leaving the customs union will have consequences. And what I saw ... in the last year, is that many of these consequences have been underestimated in the UK. Now we have to face the reality.
Further problems arose after Mr Barnier struck down any notion of a 'Canada-style' free trade agreement with Brussels, stating that the geographical proximities of a UK-EU free trade deal were "very particular" and could not work as with deals between the UK and South Korea or Japan.
The comments were followed by London's plans to bar non-English speaking migrants and unskilled overseas workers after Brexit, which the Cabinet Office arguing for further investment in automation and retaining current workforces, allowing Downing Street to keep campaign promises to reduce net immigration figures.
The UK officially left the European Union at 23:01 on 31 January with a deadline of 31 December to finalise any negotiations with Brussels and secure free trade agreements, or risk entering a 'no-deal' scenario under World Trade Organisation terms. UK Prime Minister Boris Johnson said that he would deliver Brexit 'come what may' after snap elections in early December, where UK Conservatives landed a further 80 seats in Commons.