Scotland's First Minister Nicola Sturgeon has said she is "determined" to have a second independence referendum, adding a move by Theresa May to block it would be “democratic outrage”.
It comes after Downing Street announced it is to reject calls for another vote before Brexit.
The Prime Minister's position was confirmed by Scottish Conservative leader Ruth Davidson, who said Ms Sturgeon's timetable for a referendum - between autumn 2018 and spring 2019 - would be "rejected conclusively".
Ms Sturgeon wants a referendum to coincide with the conclusion of the Brexit talks - the point at which she says the terms of the UK's deal to leave the EU will become clear.
In an interview with BBC Scotland, Ms Sturgeon said: "I am determined that I will have one on my timescale, because the will of the Scottish Parliament will be respected.
"I accept that the Prime Minister has said what she has said today.
“What I don't accept is that that position is an acceptable one, a democratic one or a sustainable one."
MSPs will vote next week on whether they will support Ms Sturgeon's request for a section 30 order from Westminster, which would be needed for Holyrood to hold a legally binding ballot.
The First Minister has insisted that the Scottish Government has a "cast-iron democratic mandate" to offer people a choice.
Earlier on Thursday she said: "If the Prime Minister refuses to engage on the terms of a referendum before Brexit takes place then she is effectively trying to block the people of Scotland having a choice over their future.
"That would be a democratic outrage.
"It is for the Scottish Parliament - not Downing Street - to determine the timing of a referendum, and the decision of the Scottish Parliament must be respected.
"It would be outrageous for the Scottish Parliament to be frozen out of the process."
But in an interview for ITV news, Mrs May said: "Right now we should be working together, not pulling apart.
"We should be working together to get that right deal for Scotland, that right deal for the UK, as I say that's my job as Prime Minister and so for that reason I say to the SNP: now is not the time."
She added that to be "talking about an independence referendum will, I think, make it more difficult for us to be able to get the right deal for Scotland, and the right deal for the UK".
She said: "More than that I think it wouldn't be fair to the people of Scotland because they'd be asked to make a crucial decision without the necessary information, without knowing what the future partnership will be or what the alternative for an independent Scotland would look like."
Asked whether Mrs May was ruling an independence referendum out altogether, or only during the period leading up to Brexit, her official spokesman said: "The Prime Minister was responding to a specific proposal from the First Minister."
The spokesman said Mrs May is hoping the Scottish Parliament will now back away from staging a vote.
Additional reporting by Press Association.