The UK government is to reject calls for a Scottish independence referendum before Brexit after Theresa May said "now is not the time".
The prime minister said the focus should be on getting the best Brexit deal for the whole of the UK.
Scottish Conservative leader Ruth Davidson said Nicola Sturgeon's demand for a vote by the spring of 2019 would be rejected "conclusively".
Ms Sturgeon said blocking a referendum would be a "democratic outrage".
Ms Sturgeon, the Scottish first minister, told BBC Scotland: "It is an argument for independence really in a nutshell, that Westminster thinks it has got the right to block the democratically elected mandate of the Scottish government and the majority in the Scottish Parliament.
"You know history may look back on today and see it as the day the fate of the union was sealed."
Ms Sturgeon has called for a referendum to be held in the autumn of 2018 or the spring of the following year, to coincide with the conclusion of the UK's Brexit negotiations with the EU.
But Mrs May said her message to Ms Sturgeon was clear - "now is not the time".
The prime minister added: "I think we should be working to get the right deal for Scotland and the UK with our future partnership with the European Union.
"It would be unfair to the people of Scotland that they would be asked to make a crucial decision without the information they need to make that decision."
The prime minister also said the country should be "working together, not pulling apart".