Ursula von der Leyen, the president of the European commission, has warned Boris Johnson not to break international law as her Brexit negotiator said he would seek answers over claims that Downing Street is planning to negate parts of the withdrawal agreement. “I trust the British government to implement the withdrawal agreement, an obligation under international law & prerequisite for any future partnership,” Von der Leyen tweeted. “Protocol on Ireland/Northern Ireland is essential to protect peace and stability on the island & integrity of the single market.” The commission president was responding to reports that the government is planning legislation to override parts of the withdrawal agreement struck last year that seek to avoid a hard border on the island of Ireland. Ahead of the start on Tuesday of the latest round of trade and security negotiations with the British government, the EU’s chief negotiator, Michel Barnier, also reiterated that the Northern Ireland protocol in the withdrawal deal was a “prerequisite for peace since the end of the conflict ... and it’s the prerequisite for a united and coherent economy for the entire island, and also to respect the single market”. “Everything that has been signed must be respected,” Barnier told France Inter radio. “We demand quite simply, and calmly, and until the end, that the political commitments in the text agreed by Boris Johnson be legally translated into this treaty. “The important thing for me is what the prime minister says and does, and what the British government itself says and does,” he said. Whether there is a new deal with the EU on trade and security or not by the end of the year, under the withdrawal agreement Northern Ireland will stay in the EU’s single market from 2021 including its rules on subsidies, known as state aid. The bloc’s customs code will also be implemented in full on goods coming into the province from the rest of the UK, requiring checks. The Financial Times reported that the government was, however, seeking to very narrowly define the obligation to notify Brussels of subsidy decisions for it to approve. Clauses in the bill could also dispense with the requirement for local businesses to file customs paperwork when sending goods into the rest of the UK. The options are reported to be contained within the internal market bill due to be published on Wednesday. Fallback legislation will be necessary anyway should the Northern Ireland assembly vote to reject the withdrawal agreement’s protocol during periodic votes, as is permitted under the treaty, and the full details of the upcoming legislation are yet to be spelled out. But the suggestion that the UK might be seeking to renege on its deal with Brussels in the event of the troubled trade and security negotiations breaking down brought swift denunciations on Monday. Ireland’s foreign affairs minister, Simon Coveney, who helped broker the original Brexit settlement, tweeted that unravelling the withdrawal agreement would be “a very unwise way to proceed”. The deputy first minister of Northern Ireland and vice- president of Sinn Féin, Michelle O’Neill, said on social media: “As the Brexit negotiations between the EU and British government enter their eighth round this week in London, any threats of a rollback on the Irish protocol would represent a treacherous betrayal which would inflict irreversible harm on the all-Ireland economy, and [Good Friday agreement].” The UK’s chief negotiator David Frost and Barnier will re-engage in negotiations in London on Tuesday with expectations low for any progress. The two sides are at loggerheads on future access to British waters for European fishing fleet and the UK’s plans for its domestic subsidy regime from next year. The EU is pushing for a linkage between its rule book any future UK system for controlling the level of domestic subsidies to ensure there is fair competition between British and European businesses. Downing Street has insisted there is no precedent for such a linkage between subsidy rules in free trade deals. With common ground proving difficult to find, Johnson has issued a statement effectively setting a five-week deadline for agreement to be found. Downing Street has been concerned that the EU planned to stretch talks deep into October, prolonging uncertainty. The prime minister said: “There needs to be an agreement with our European friends by the time of the European council on 15 October if it’s going to be in force by the end of the year. So there is no sense in thinking about timelines that go beyond that point. If we can’t agree by then, then I do not see that there will be a free-trade agreement between us, and we should both accept that and move on.” In response, Germany’s ambassador to the UK, Andreas Michaelis, called for calm and dismissed claims that the EU’s negotiating aims involved surrendering British sovereignty. He tweeted: “After the Brexit remarks of PM I wonder if can still claim being an independent country. Is party to hundreds of international treaties. Underlying compromises have certainly not eroded our sovereignty. Same would hold true of a Brexit deal. Let’s get on with it!” In Brussels, EU diplomats were weary of what is being seen as further posturing by Downing Street. “We all remember what happened the last time when someone wanted ‘to put a tiger in a tank and ‘add a bit of oomph in the negotiations”, one said. “Message spinning and posturing is all fine, but it doesn’t really bring us an inch closer to a solution. There has been absolutely no movement from the British side in the talks yet. If this approach doesn’t change quickly, we will be on a road to no-deal with all its negative economic consequences.” The diplomatic source added: ““More and more people have come to the conclusion that Brexit ideology trumps Brexit pragmatism in the UK government. “If No. 10 really wanted to jump off the Brexit cliff edge for ideological reasons, there would be absolutely no way for the EU to stop this. If, on the other hand, the UK’s approach became more pragmatic and realistic, there would probably be a good chance to save the negotiations and agree on a deal in October.”