Brussels has urged Westminster to throw out the “illegal” attempt by Boris Johnson to unilaterally rewrite the post-Brexit arrangements for Northern Ireland, as the EU launched fresh legal action against the UK. Maroš Šefčovič, the EU’s Brexit commissioner, said he expected parliament to make the case for negotiation over conflict, as he relaunched a frozen case for past breaches of the Brexit deal and started two new claims over undelivered treaty obligations. “Let’s call a spade a spade: this is illegal,” he said of the prime minister’s plans. As a result of the EU’s initial response, the UK will need to justify its past failures to check imports from Britain to Northern Ireland within two months, or face potentially huge fines from the European court of justice. Letters of formal notice of action will also be sent over the past failures to establish border posts and share data. However, Brussels held back on taking targeted action over the new legislation tabled on Monday – which proposes rewriting the Northern Ireland protocol – until the bill becomes law, a process that could take 18 months or longer. Šefčovič suggested that suspending parts of the trade deal, at a potentially heavy cost to British consumers and industry, could not be ruled out but he used a press conference in Brussels to appeal to MPs and peers to stop the Johnson government from trashing the UK’s reputation and provoking a trade war. “We are now bringing the argument also into the debate, which I’m sure will be in the House of Commons and House of Lords, that there is a better way to solve these issues than having these legal disputes with the EU,” he said. In response to suggestions that the UK could ignore ECJ rulings against it, given Johnson’s plans to rip up past agreements, Šefčovič said Britain’s reputation was on the line. He said: “Not respecting the European court of justice rulings will be just piling one breach of international law on another. “This way forward, is it compatible with the proud British traditions of upholding and respecting the rule of law and international law in that regard? So that’s, I would say, the political question I’m throwing up and, of course, how other potential partners would look at the UK when they will be negotiating agreements. “Will they be changed in one year, in two years? Will they really stick? Will they be respected?” There is anger in EU capitals at Downing Street’s approach, with Ireland’s taoiseach, Micheál Martin, describing his most recent call with Johnson as the worst of his political career. Miguel Berger, Germany’s ambassador to the UK, warned on Wednesday that countries such as China and Russia would be looking “very closely” at Britain’s stance on international law, describing the issue as “very serious”. Johnson has said the draft legislation rewriting the arrangements for Northern Ireland was both “trivial” in its scope and also legal because of the “necessity” to breach treaty obligations in the face of a risk to the balance of the Good Friday agreement. In response to the EU’s legal action, a government spokesperson said on Wednesday it was “disappointing”, but that Brussels had been intransigent in refusing to change the text of the Brexit deal. Šefčovič dismissed those claims. “Let there be no doubt there is no legal nor political justification whatsoever for unilaterally changing an international agreement,” he said. “Opening the door to unilaterally changing an international agreement is a breach of international law as well.” Under the protocol agreed by Johnson in 2019, Northern Ireland in effect stays in the single market and the EU’s customs rules are applied down the Irish Sea to avoid a border on the island of Ireland. Under the new legislation, the government would scrap checks for firms selling goods from Great Britain destined for Northern Ireland rather than the EU. The government envisages the creation of a “green lane” of fewer checks for those selling goods heading for Northern Ireland and a “red lane” with existing checks for goods destined for EU countries. EU officials said they did not see any major differences between this proposal and those tabled by the European Commission for an “express lane”. However, the legislation would allow companies in Great Britain exporting to Northern Ireland to choose between meeting EU or UK standards on regulation, which are expected to increasingly diverge. EU officials said this posed a risk to the single market and that the current arrangements and the failure to implement controls had already been a spur to smugglers. Further measures include changing oversight of trade disputes so that they are resolved by independent arbitration rather than the European court of justice. “Removing the role of the court of justice is out of the question,” the EU official added. Brussels remains open to further talks and two papers were published highlighting the flexibilities that are on offer to ease the burden of checks on goods entering Northern Ireland. An official said: “We know that the UK suggests that, as a result of protocol, for a single lorry moving from Sainsbury’s in Great Britain to Sainsbury’s in Northern Ireland an enormous amount of paperwork has to be filled in, and what we’re publishing today are documents which say, no, actually it is three pages for an entire truck.”