Boris Johnson says there is a "strong possibility" the UK will fail to strike a post-Brexit trade agreement with the EU.
The prime minister said "now is the time" for businesses and the public to prepare for that outcome, although negotiators would continue talks.
He added that negotiations were "not yet there at all".
It comes after his meeting on Wednesday with the president of the EU Commission failed to reach a breakthrough.
Weeks of intensive talks between officials have failed to overcome obstacles in key areas including competition rules and fishing rights.
In his first public comments since Wednesday night's dinner meeting in Brussels, Mr Johnson said British negotiators were going to "go the extra mile," and he was prepared to travel to Paris or Berlin for talks if necessary.
But he said the EU wanted to keep the UK "locked" into its legal system, or face punishments such as taxes on imports, which had "made things much more difficult".
"At the moment, I have to tell you in all candour the treaty is not there yet and that was the strong view of our cabinet," he said.
He added that "looking at where we are," it was vital the UK prepares for the "Australian-style option" of not having a free trade deal with the EU.