Nicola Sturgeon hit out at the Prime Minister after he conceded that reaching an agreement for the UK to leave the European Union by 31 October was now "touch and go".
The Conservative leader had previously insisted it was a "million to one chance" the UK would be forced into a no-deal Brexit.
The First Minister said: "He's gone in the matter of a few weeks from saying that a no-deal Brexit in his view was a million to one chance, to now saying it's touch and go which suggest to me he's making it up as he goes along.
"That's not acceptable because the consequences of this are so serious for our economy, for our society, for the livelihoods and living standards of people the length and breadth of the UK that that's just not good enough. "
She added: "The House of Commons returns to session next week and the SNP MPs will be working hard with other parties to find whatever way we can of stopping no deal in its tracks because that would be catastrophic.
"Scotland didn't vote for Brexit in any form but a no-deal Brexit would be particularly damaging and we'll do everything we can to stop it."
Holyrood will next week step up its opposition to a no deal Brexit when MSPs return from the summer recess.
Scottish Greens and Liberal Democrat MSPs have already signalled they will support the Government's motion on this, saying "the Scottish Parliament agrees that the UK should in no circumstances leave the EU on a no-deal basis", ensuring that this will pass.
It comes after Mr Johnson, who has pledged to take Britain out of the EU by 31 October "do or die", held a series of talks with key European leaders.
And following those discussions he refused to repeat his previous assertion that the odds of a no-deal outcome were a million to one.
"It all depends on our EU friends and partners, I think in the last few days there has been a dawning realisation in Brussels and other European capital what the shape of the problem is for the UK," the Prime Minister said.
"I think it's going to be touch and go but the important thing is to get ready to come out without a deal."