A post-Brexit trade and security deal could be sealed as early as this week after Boris Johnson made a key concession at the weekend but the pathway to agreement remained “very narrow”, Michel Barnier told ambassadors and MEPs in Brussels. The EU’s chief negotiator said the prime minister’s acceptance of the need to ensure that there was fair competition for British and European businesses as regulatory standards diverged over time had unlocked the talks despite difficult issues remaining. “For the first time,” Barnier said, the UK government had “accepted a mechanism of unilateral measures”, such as tariffs, where there were “systemic divergences which distort trade and investment”. “But this mechanism needs to be credible, effective and quick,” he added. “We are working on this.” Sitting on a panel next to the French president, Emmanuel Macron, in Paris, the European commission president, Ursula von der Leyen, said there was “movement”. “That is good,” she said. “We are talking about a new beginning with old friends. We are on the very last mile to go. But it is an essential one. We want a level playing field, not only at the start, but also over time and this is the architecture we are building.” “We are fine about the architecture itself but the details in it, do they really fit?” she said. “These are crucial points. If the UK wants a seamless access to the single market of the EU – and it’s the largest one in the whole world – they are welcome, but they have to play by our rules.” A Downing Street spokesman said the government was “committed to try and bridge the gaps that still exist”. “We’ve been clear that we will continue to work, and hope to reach a free trade agreement,” he added. “Obviously, no deal is a possible outcome. Time is in very short supply and it has been for some time.” A UK source said they would be vigilant to ensure that Brussels did not introduce “dynamic alignment” of EU and UK regulatory standards. Despite positive signs on the so-called level playing field provisions, Barnier claimed talks over a deal on fishing had gone backwards, raising the risk they could run deep into December. “The next few days are important, if a [UK-EU deal] is to be in place on 1 January, 2021,” Barnier tweeted. “Fair competition, and a sustainable solution for our fishermen and women, are key to reaching a deal.” He complained that the UK believed that as it had made efforts on the level playing field, then the EU should reciprocate with concessions on fishing quotas and the frequency of negotiations. “But it does not work like this,” he said, according to sources. Barnier told MEPs the government had tabled a paper on fisheries on Monday, only to take it off the negotiating table on Thursday and that Downing Street was seeking to “renationalise” fishing boats by insisting vessels under the UK flag should be majority British-owned. UK sources said the flow of papers in the talks was a normal part of the negotiation process. “We have always been absolutely clear that the UK will have control over access to our waters and better deal for UK fishing communities – there is simply no truth in idea that we have backtracked,” the source added. The UK’s move on the so-called “evolution clause”, ensuring there is recourse to unilateral tariffs should standards significantly diverge, also left work to be done on how it would work in practice, Barnier said. According to EU sources, he explained that there were three scenarios: a deal struck by the end of this week allowing for ratification by the European parliament on 28 December; a breakdown in the talks; or agreement being found at the end of the year and the deal being “provisionally applied” to avoid a no-deal exit, with MEPs giving their consent in 2021. Alternatively contingency measures could be arranged to bridge the gap between a no-deal and ratification. The suggestion that the UK had made a key concession riled officials in London, who hit out at what they described as “inaccurate briefings” by the Brussels negotiators. “Talks remain difficult and we have not made significant progress in recent days, despite efforts by the UK side to bring energy and ideas to the process,” a UK government source said. “The inaccurate briefings from the EU side in recent days have made a difficult discussion even more challenging in the short period of time we have left.” France’s Europe minister, Clément Beaune, said on Monday the talks would have to progress rapidly. “It will be very difficult to go beyond the end of the week,” he said. “Not just because of ratification, but because we need two weeks to organise. Companies are getting anxious. We cannot get to 5pm on 31 December without companies knowing what will happen the next day.” Germany’s foreign minister, Heiko Maas, said, however, that there was no rush. “As long as an agreement is possible, we will keep negotiating,” he said. An EU diplomat said: “There might now be a narrow path to an agreement visible – if negotiators can clear the remaining hurdles in the next few days. There has been some progress in the negotiations over the last few days, but – sometimes substantial – gaps still need to be bridged in important areas like fisheries, governance and level playing field. “Success depends on whether London also wants a fair deal and is ready to accept the inherent trade-offs.” The sticking points on fishing remain the UK demand on annual negotiations and the amount of catch that will be repatriated to British vessels. The UK is also pushing back against the inclusion of fisheries within the scope of sectors that could have tariffs applied to them if there is divergence in environmental, social and labour standards over time.