Mr Johnson wants the backstop – the contingency plan to prevent a hard border with Ireland – removed from the Withdrawal Agreement thrashed out with the European Union by predecessor Theresa May.
He acknowledged that the EU was “very hard over against it” and persuading Brussels to change would “take some time”, adding that the UK would have to prepare for the prospect of a no-deal Brexit.
Speaking to reporters in Devon on a visit to a hospital, Mr Johnson said the “mood music” on his visits to Germany and France was “very good”.
“They could see that we want a deal, they can see the problems with the backstop.
“Clearly, Angela Merkel thinks that the solutions can be found within 30 days – actually what she meant was if you can do it in two years, you can certainly do it in 30 days.
“But I want to caution everybody, OK? Because this is not going to be a cinch, this is not going to be easy. We will have to work very hard to get this thing done.”
He insisted there were “lots of ways” to achieve frictionless trade at the border in Northern Ireland.
“But to persuade our EU friends and partners, who are very, very, very hard over against it, will take some time,” he said.
“I’m afraid we will have to prepare to come out without an agreement and we can do that, we are very confident that we will be OK because we will have all sorts of preparations in place.”
He added: “We are making progress but I am just telling people not to hold their breath, because I have seen the way these Brussels negotiations work.”
Meanwhile, the Government said it was determined British cities and communities will still receive “similar” funding if EU cash is withdrawn after Brexit.
Jake Berry, minister for the so-called Northern Powerhouse, claimed British versions of structural funds from the EU would help the UK flourish and there would be new trading opportunities after Brexit.