Nicola Sturgeon has not ruled out staging a second Scottish independence referendum without the consent of Westminster as she said "various options" are open.

The First Minister said her preferred option is to obtain a Section 30 order from Westminster which would allow her to hold a "legal, agreed and consensual referendum".

Theresa May has already dismissed that option, saying on Thursday that now is not the right time for another ballot.

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Ms Sturgeon told STV News on Friday: "Just because the Prime Minister has said No does not mean I immediately scurry off and say 'well, that's that'."

She was speaking as a leading academic suggested Westminster could still have to negotiate with the Scottish Government if it stages a consultative referendum.

Ailsa Henderson, a professor of political science at Edinburgh University, told a fringe meeting at the SNP spring conference in Aberdeen: "If there is a clear majority in response to a clear question expressing a preference, then both sides have a duty to negotiate."

She was pressed on whether that would apply if a referendum was consultative, not legally binding, and added: "It's a slightly different constitutional situation, but the basic principle holds."

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In her interview with STV News, Ms Sturgeon insisted she would "continue to pursue the path" she had set out when she announced plans to win the backing of MSPs to request a Section 30 order from Westminster.

Such a vote should take place between autumn 2018 and spring 2019, the SNP leader has said.

Ms Sturgeon stated: "If the Prime Minister does not agree with that timescale then I think the onus is on her to say when she thinks would be the right time and we can have a discussion.

"The next step I am going to take is to get the view and the vote of the Scottish Parliament next week because it is the body that can give me the authority to formally pursue a section 30 order. If it does, that is what I will do.

"Just because she (the Prime Minister) says something does not mean she is right nor does it mean she is in a sustainable position, so I am not going to get pushed into talking about - to use the old referendum terminology 'plan Bs' - when I am pursuing a plan A which is democratic, fair and right. And has a mandate and the backing of the Scottish Parliament."

Asked repeatedly if a consultative referendum is on the table, she said there are "various options".

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But she insisted: "My intention is that we have a referendum on the same basis we had as the one in 2014."

The September 2014 vote was held after the Scottish and UK governments both signed the Edinburgh Agreement, allowing the vote to take place and committing both sides to respect the result.

Scottish Government Brexit Minister Mike Russell also said a formal request for a Section 30 order "is the right thing to do" - although he added: "If the envelope is returned unopened we are in a different position."

Speaking at the Edinburgh University fringe event, Mr Russell said: "We have to think our way through this very carefully. I'm prepared to think my way to next Wednesday evening, which is when the vote will be taken in the Scottish Parliament on the issue of the Section 30 request. Then we see where we go from there."

Meanwhile, Professor Richard Wynn Jones of Cardiff University said it could be "politically productive" for Mrs May to have a conflict with the Scottish Government over the staging of another vote.

Research carried out by political parties south of the border "suggests Nicola Sturgeon was very unpopular with lots of the English electorate", he said.

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Speaking about the Prime Minister, he added: "This is potentially productive for her politically, even though it's high stakes, it may well be helpful for her politically, and it's important to remember that."

Scottish Conservative deputy leader Jackson Carlaw branded the possibility of a consultative vote on independence "a nonsense Catalan castanets consultative referendum".

The Tory MSP said: "The SNP's only reason for existing is to rip Scotland out of the UK.

"That's why the party refuses to rule out any means of achieving this, including a nonsense Catalan castanets consultative referendum.

"Such a process would make Scotland the laughing stock of the world.

"The 2014 referendum was legal and fair, but a wildcat version - as so many SNP members seem to want - would be anything but.

"It appears Nicola Sturgeon's half-baked referendum plans are descending into chaos."