The future of Britain’s relationship with the rest of Europe will hang on the success of a dinner between Boris Johnson and Ursula von der Leyen in Brussels on Wednesday, it has emerged, as the EU’s chief negotiator warned the chance of a Brexit deal was now “very slim”. Downing Street said the prime minister would join the European commission president at its Berlaymont headquarters on Wednesday evening, where the leaders would seek to break the Brexit impasse over a three-course meal. Johnson will take prime minister’s questions in the House of Commons before flying from RAF Northolt to Brussels to dine with Von der Leyen, armed with a memo from his chief negotiator, David Frost, on the remaining obstacles to a historic trade and security deal with the EU. He will return to Downing Street after the discussions. Von der Leyen tweeted on Tuesday: “I look forward to welcoming UK prime minister Boris Johnson tomorrow evening. We will continue our discussion on the partnership agreement.” No 10 sources pushed back on rumours that Johnson had wanted to delay the meeting to allow the 27 EU heads of state and government to intervene in the discussions when they gather at the European council summit in the Belgian capital on Thursday. They insisted the prime minister “categorically wants to avoid” such a scenario. A deal is not expected to be sealed at the dinner but Downing Street said if there was progress and goodwill, the troubled negotiations could be expected to resume on Friday after the council summit. EU leaders will be informed of any progress when they meet, or could trigger the bloc’s no-deal contingency measures – keeping planes in the sky and borders safe – if the dinner ends in failure. “It will continue past [Wednesday] if they can make some headway and it’s worth it,” one senior UK official said. Downing Street said it was open to continuing talks into the weekend and beyond, but stressed nothing could go on past 1 January. “We have been clear that the future relationship needs to be concluded by the end of the year and negotiations won’t continue in the new year,” a No 10 spokesman said. “That’s been our position throughout.” Speaking in private to EU ministers on Tuesday, the bloc’s chief negotiator, Michel Barnier, who has so far resisted calls from some member states to trigger the commission’s no-deal contingency plans, admitted that such measures could soon be necessary given the small chances of success. “We are close to the moment of needing urgent measures which means a contingency plan for no deal,” Barnier said, according to EU sources. “The basis of our future cooperation with the UK is more important than rushing now. We cannot sacrifice our long-term interests for short-term political goals.” Johnson and Von der Leyen agreed on Monday evening to hold a make-or-break meeting with just three weeks to go until the UK leaves the EU’s single market and customs union, with or without a deal. Frost travelled back to London on Tuesday night to brief Johnson after finalising an agenda with his EU counterpart comprising the key remaining differences to be discussed in the summit. A UK government official confirmed they were expecting Frost to “resume negotiations over the coming days” as long as the dinner between the two leaders showed there was scope for change. “It’s clear that some political impetus will be required for the talks to make any more progress,” the source said. “But we must be realistic that an agreement may not be possible.” Barnier had said earlier in the week that he could not negotiate beyond midweek due to a Commons vote on Wednesday on new legislation which would again break international law by overriding clauses in the withdrawal agreement. But Downing Street’s decision on Tuesday to abandon all the Brexit clauses relating to Northern Ireland in both the internal market and finance bills removed that obstacle to more talks. Maroš Šefčovič, the commission’s vice-president, who has been working with the Cabinet Office minister, Michael Gove, said the move “removed one big obstacle”. He said: “I hope this will create positive momentum for the discussions on the free trade agreement. We’re still very far apart and we’re not hiding this from anyone. We’ve removed one big obstacle from the way and I hope we will see the positive results also coming from this very complex negotiations.” The European commission’s chief spokesman said Johnson and Von der Leyen would not be sealing a deal but trying to find reason to “move forward hopefully with negotiations, which could hopefully continue after that … This is uncharted territory – we’ll have to see how this meeting goes.” Earlier on Tuesday, Johnson said securing a deal would be “very, very difficult”. “You’ve got to be optimistic, you’ve got to believe there’s the power of sweet reason to get this thing over the line,” he said. “But I’ve got to tell you it’s looking very, very difficult at the moment.” On Tuesday night a UK government source similarly downplayed prospects of a deal, saying: “It’s clear that some political impetus will be required for the talks to make any more progress. “If we can make progress at a political level it may allow Lord Frost and his team to resume negotiations over the coming days. But we must be realistic that an agreement may not be possible as we will not compromise on reclaiming UK sovereignty.” Johnson’s cabinet met on Tuesday morning and a spokesman said it was unified around the current position. “We obviously want to continue to try to reach a free trade agreement, that has been our position throughout,” the spokesman said. “But the cabinet also agrees that any agreement must respect our sovereignty and allow us to take back control of our money, laws and borders.” Germany’s European affairs minister, Michael Roth, said the result of the summit would depend on the UK government’s “political will” to seal a deal. He said: “What we need is political will in London. Let me be very clear: our future relationship is based on trust and confidence. It is precisely this confidence that is at stake in our negotiation right now.” Clément Beaune, France’s European affairs minister, said his government would not be rushed into an agreement that was not in its interests. “If when we look at it we see that it is not as good as not having a deal, we will not hesitate to veto it – as is the case with every other European country who will do this evaluation.”