Senior Nationalists gathering in Aberdeen for the start of the party’s Spring conference tomorrow have warned that the move will backfire and drive up support among Scots for leaving the UK.
Ms Sturgeon described Mrs May’s intervention as “winding the clock back to the bad old days of Margaret Thatcher.”
The Scottish Parliament is to vote on the staging of a second referendum next week, with the minority SNP Government’s proposal likely to be passed with the support of the Greens. Ms Sturgeon insisted that blocking such a move could only boost her plans for separation.
Ms Sturgeon told STV News: “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.”
Power over the the constitution is reserved to the UK and Mrs May refusal to back a section 30 order - which was in place before the 2014 referendum - means a legally binding vote cannot now be be held.
The SNP leader’s keynote address to her conference delegates on Saturday will now be eagerly anticipated, with supporters keen to hear how the Scottish Government will respond to their referendum plans being blocked.
Ms Sturgeon added yesterday: “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 insists that she has a mandate to push for a second referendum after the Brexit vote saw a majority of Scots voting to Remain, but the weight of votes south of the border swung the outcome swing in favour of Leave. Such a scenario was set out in the SNP manifesto going into last year’s Scottish election as “material change” in circumstances which would allow the party to push for a quickfire second referendum.