Theresa May has accused the SNP of being "divisive and obsessive" nationalists as the feud between the Prime Minister and the Scottish Government over a second independence referendum intensified.
Mrs May accused First Minister Nicola Sturgeon and her party of being in a "muddle" about their plans and warned that breaking up the UK would be "bad for us all".
Her intervention, at the Conservative spring forum in Cardiff, came as SNP deputy leader Angus Robertson insisted there was "no doubt" that a second referendum would take place - even though Mrs May has ruled out a vote on independence before Brexit has been finalised.
Downing Street dismissed the suggestion that the SNP could hold a legal referendum without the PM's approval between autumn 2018 and spring 2019, the timetable set out by Ms Sturgeon.
The countdown to Brexit will begin during the next two weeks when Mrs May triggers Article 50.
The Prime Minister went on to accuse the SNP of "tunnel vision" and seeking to exploit the vote to leave the European Union.
Theresa May went on to say Brexit negotiations would be "vital" for every citizen, business and community; saying getting the right deal should be the focus.
She hit out at the "divisive and obsessive nationalisms" of the SNP and Plaid Cymru, and mocked Ms Sturgeon's party over its approach to EU membership.
Earlier, Mr Robertson set out the potential for a constitutional crisis if Mrs May blocked a fresh independence ballot.
If Holyrood on Wednesday backs Ms Sturgeon's bid for a fresh ballot when the terms of the UK's Brexit deal are known, it would be "undemocratic and totally unacceptable" for this to be denied by Westminster, he said.
At his party's spring conference in Aberdeen, Mr Robertson said: "The Tories' argument is not about process, it is about their desperate desire to prevent anyone having the chance to reject the hard-right Brexit that they are so wedded to.
"The truth is, it should not be for either Theresa May or the Scottish Government to decide Scotland's future -that choice belongs to the Parliament and the people of Scotland and it is one this party will never, ever shy away from."