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.

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."