A move to block a second independence referendum would be a "democratic outrage", Nicola Sturgeon has said.
Downing Street is to reject calls for another vote before Brexit after Theresa May said "now is not the time".
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".
The First Minister 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.
"The Scottish Government has a cast-iron democratic mandate to offer people a choice and that mandate must be fulfilled."
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.
MSPs will vote next week on whether they will support her request for a section 30 order from Westminster, which would be needed for Holyrood to hold a legally binding ballot.
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."
At a news conference in Edinburgh shortly after Mrs May's statement, Scottish Secretary David Mundell confirmed the UK Government "will not be entering into discussions or negotiations about a section 30 agreement and any request at this time will be declined".
Speaking alongside Mr Mundell, Ms Davidson said: "We reject conclusively the timetable for a referendum set out by the Scottish Government for a key reason - because it is unfair to Scottish voters.
"We've just come through a referendum campaign where a key complaint among many people was that they did not have the necessary information to help them make an informed decision.
"If we were to keep to the First Minister's timetable this is exactly what would happen in Scotland too. On the most important political decision a country can make we would be voting blind.
"If the SNP insists on pressing ahead, we will argue that a referendum cannot happen when the Scottish people have not been given the opportunity to see how our new relationship with the European Union is working.
"And until there is clarity over the alternative.
"And we will maintain that it should not take place when there is no clear public or political consent for it to happen."
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