Boris Johnson has warned that securing a deal at an imminent Brexit summit in Brussels will be “very difficult” but backed “the power of sweet reason to get this thing over the line”. The prime minister, who agreed during a phone call on Monday to meet the European commission president, Ursula von der Leyen, in the Belgian capital, said he wanted an agreement. “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,” Johnson said. “But I’ve got to tell you it’s looking very, very difficult at the moment. “We’ll do our level best, but I would just say to everybody – be in good cheer, there are great options ahead for our country on any view. But the key thing is, on 1 January, whatever happens there’s going to be change and people need to get ready for that change.” Asked if he would try to do a deal right up until the wire, Johnson told reporters: “Yeah, of course. We’re always hopeful but you know there may come a moment when we have to acknowledge that it’s time to draw stumps and that’s just the way it is.” The prime minister insisted he would be willing to leave the transition period without a deal. He said: “We will prosper mightily under any version and if we have to go for an Australian solution then that’s fine too.” Australia and the EU do not have a free trade deal and there are tariffs on goods, including 48% on lamb and 84% on beef. In a joint statement on Monday, the two leaders agreed in the coming days they would hold a make-or-break meeting. Sources on both sides pointed to Wednesday or Thursday morning as the most likely times. The UK’s chief negotiator, David Frost, will travel back to London on Tuesday night to brief Johnson after finalising an agenda with his EU counterpart, Michel Barnier, of the key remaining differences to be discussed in the meeting with Von der Leyen. No timings for a meeting have been agreed yet. No 10 suggested the discussions would be a forum for genuine negotiation, a suggestion which had been dismissed by the EU. “The prime minister recognises that we have probably taken technical talks as far as they can,” a Downing Street spokesman said. “The in-person meeting will be to see if there is a political way through. Nothing has been agreed yet in terms of the details of the meeting.” 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.” EU leaders will meet on Thursday, when they could sign off on an agreement or trigger their preparations for a no-deal outcome, including temporary legislation to keep planes in the air. Frost, joined commission officials on Tuesday morning to prepare a document for the two leaders on the outstanding issues over fair competition, EU access to British fishing waters and the terms of a dispute mechanism in the event of treaty breaches. 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: “It is good that every effort is undertaken to find a sustainable and good solution. We want to reach a deal but not at any price. 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.” Roth was speaking at a virtual meeting of EU ministers preparing for Thursday’s summit of the 27 member states’ leaders. Clément Beaune, France’s European affairs minister, said his government would not bow to time pressure with less than three weeks to go before the UK leaves the single market and customs union with or without a deal. He said: “The truth, the facts, is that there is still a negotiation ongoing … Concretely, it’s complicated and we don’t want to give in to a form of pressure from the British.” He said fisheries was a major issue. “That’s 6,000 jobs in France. It’s a big deal. There is no reason that just because it’s important for the British, we say ‘OK, we give up, you deny us access to your waters and we will go elsewhere’. That’s not possible. We will make efforts, compromises, yes, we have said that honestly to the French fishermen. But sacrifice our fishermen – no. And the British know it … “Yes, we are a bit fed up. But we cannot say that we will slam the door and walk away because we’re tired of it all.” Beaune repeated France’s threat to veto the deal if it failed to protect French fishermen’s rights to operate in British seas and offered the UK the opportunity to undercut EU standards. He said: “We will look at the deal when it’s on the table and we will analyse whether or not it defends our 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. “I do not want to declare these talks a failure. I think we still have some time to negotiate – a few days – and after we have to say clearly, because it’s important for our fishermen, our businesses, we have to say yes, or no.”