Nicola Sturgeon has insisted it would be "wrong, unfair and utterly unsustainable" for Westminster to deny Scotland a second independence referendum if MSPs back her call for another ballot to be held.
The First Minister said it was "'entirely legitimate'' for the UK Government and opposition parties at Holyrood to ''robustly'' oppose independence.
But with the Scottish Parliament expected to back her call for another referendum in a vote on Wednesday evening, she insisted she has an "unquestionable democratic mandate" for such a vote to take place.
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Her comments come as Ms Sturgeon and the Prime Minister are engaged in a constitutional stand-off.
The SNP leader wants another vote to take place between autumn next year and spring 2019, when she says there will be clarity over the outcome of the Brexit negotiations.
Theresa May has declared ''now is not the time'' for another ballot, indicating the UK Government will reject the SNP's preferred timetable.
With the PM due to trigger Article 50 and start the formal two-year-long Brexit process on March 29, Ms Sturgeon told MSPs: "As a country, we can't avoid change. But we can choose what kind of change we want."
The First Minister spoke out at the start of a two-day debate at Holyrood in which MSPs are considering a motion to mandate the Scottish Government to take forward discussions with the UK Government on the details of a section 30 order - the mechanism to transfer the legal powers for a vote.
Scottish Conservative, Labour and Liberal Democrat politicians have already made clear they will seek to block another referendum.
But the pro-independence Scottish Greens, who have six MSPs, will give the First Minister the support she needs for her motion to be passed.
A total of 62% of Scottish voters backed the UK remaining part of the EU in June 2016 and the SNP manifesto for last year's Holyrood elections made clear another ballot on independence should take place if there were a "material change in circumstances" from the previous ballot in 2014.
The example cited for that was for Scotland to be removed from the EU against its wishes.
Ms Sturgeon said: "In the circumstances we now face, for the UK Government to stand in the way of Scotland even having a choice would be, in my view, wrong, unfair and utterly unsustainable."
The First Minister has already said she was forced to move for a second referendum as a result of Westminster intransigence, with Ms Sturgeon still waiting for a formal response to "compromise" proposals which would have kept Scotland in the single market and handed more powers to Holyrood.
"The UK Government is taking decisions entirely unilaterally that I and many others believe will be deeply damaging to our economy and standing in the world," she said.
Ms Sturgeon added: "The decision about the kind of country we are and the path we take can only be made by the people of Scotland."
Scottish Conservative leader Ruth Davidson insisted: "Most people in Scotland are sick to death of the SNP's games.
"They don't want another referendum any time soon, just three years after the last one."
Ms Davidson questioned how the SNP could "demand another referendum when they still cannot answer the basic questions on their own proposition" on issues such as currency and EU membership.
The Tory said: "There cannot be a referendum until people know what they're voting for.
"Until the Brexit process is complete and they know both what the UK and what independence looks like"
She concluded: "This referendum may be the First Minister's priority. It is not mine. Nor that of my party.
"We say let this Parliament focus on the issues we were elected to deliver on - better schools, a sustainable NHS, a growing economy.
"And a strong Scotland - as part of a strong United Kingdom."
Labour and the Liberal Democrats were also united in opposing the Scottish Government
Scottish Labour leader Kezia Dugdale said Ms Sturgeon was focused on "the only thing that has ever really mattered to the SNP'' - independence.
Ms Dugdale said: ''Nicola Sturgeon wakes up every single day thinking of ways to engineer another referendum.
''Because leaving the UK is the only thing that matters to her. 'It isn't improving education in Scotland. It isn't lifting children out of poverty.
''It's independence. That will always come first and it always has.''
She added: ''Brexit isn't the motivation for another referendum - it's just the latest excuse."
Willie Rennie, the Scottish Liberal Democrat leader, argued that having another vote in independence in response to Brexit would bring ''chaos on chaos'' and would ''divide'' families, communities and friends.
He said: ''Division with Europe is not resolved by division in the UK. The response to hard Conservative Brexit is not hard SNP independence.
''We do not mount chaos onto chaos of Brexit with the chaos of independence.
"We do not respond with a break from Europe with a break from the UK.''
But Scottish Greens co-convener Patrick Harvie, whose party backs the call for another referendum, said: ''It is absurd to suggest that we should not respond to and react to the fundamentally-changed circumstances we now find ourselves in.''
He added: ''The situation is changed not only by the EU referendum result, but by everything the UK Government has done with it.
''The recklessness of holding that referendum to resolve their own internal squabbles, the utter lack of a plan ... and the disrespect shown to Scotland since then.''
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