SNP depute leader Angus Robertson has said there is "no doubt" a second referendum on Scottish independence will take place, despite Theresa May insisting "now is not the time" for such a vote.

Mr Robertson said if Holyrood backs Nicola Sturgeon's bid for a fresh ballot on Wednesday it would be "undemocratic and totally unacceptable" for this to be denied by Westminster.

Speaking at the start of the SNP spring conference in Aberdeen, he said: "Let there be no doubt - Scotland will have its referendum and the people of this country will have their choice. They will not be denied their say."

He added: "Scotland's referendum is going to happen and no UK Prime Minister should dare to stand in the way of Scotland's democracy."

The conference is taking place just days after the Scottish First Minister stunned Westminster with her announcement that she is to seek the backing of MSPs for a Section 30 order, which would give Holyrood the right to hold a legally binding referendum.

Ms Sturgeon is planning for that vote to be held sometime between autumn 2018 and spring 2019, when the terms of the UK's Brexit deal are known.

The Prime Minister's "panicked response" to that, when she said there should not be a vote at this time, shows that "the Tories are simply scared of the people's choice", Mr Robertson said.

The SNP's Westminster leader told the conference: "The Tories' argument is not about process, it is about their desperate desire to prevent anyone having the chance to reject the hard-right Brexit that they are so wedded to.

"The truth is, it should not be for either Theresa May or the Scottish Government to decide Scotland's future - that choice belongs to the Parliament and the people of Scotland and it is one this party will never, ever shy away from."

With the UK having voted to leave the European Union and Mrs May set to trigger Article 50 later this month, formally starting the Brexit process, Mr Robertson said: "We cannot drift along for the next two years and hope for the best.

"We face the real prospect of right-wing Tory governments until at least 2030 and being dragged out of the EU and single market with all the damage to our economy and society that will cause.

"If the Prime Minister refuses to engage on the terms of a referendum before Brexit takes place, then she is effectively trying to block the people of Scotland having a choice over their future. That would be a democratic outrage.

"If Scotland can be ignored on an issue as important as our membership of the EU and the single market, then it is clear that our voice and our interests can be ignored at any time and on any issue."

He insisted: "So when the terms of Brexit are known - and not before - we will give the people the choice over the direction Scotland should take - before it is too late to change course.

"Before people make that choice we will set out the challenges and opportunities of independence. In particular we will set out how to secure our relationship with Europe, build a stronger economy, a fairer society, and forge a genuine partnership of equals across these islands."

He hit out at the "hard Brexit fixation of the Tories" and, with the Scottish Parliament expected to back Ms Sturgeon's call for a Section 30 order, Mr Robertson said: "The big question for Theresa May is: does she respect Scottish democracy, does she respect the cast-iron mandate of the Scottish government?

"If the Scottish Parliament decides that there should be a referendum after the Brexit negotiations are concluded, but before Scotland is dragged out of the EU, will she respect that?"

He challenged the Prime Minister, saying: "Will she give a clear and unambiguous statement today that she will respect the wishes of Scotland's national parliament?

"If she does not, this will be the proof that the Tories have returned to the bad old days of Thatcher's Britain running roughshod over Scotland, and we will not accept that."

Mr Robertson added: "Nobody is proposing that there should be a referendum held right now. The Prime Minister knows that and everyone understands that.

"But if it's only fair to be able to conclude Brexit negotiations before a referendum, then it's only fair that the people should have their say in the months after that, before the UK leaves the EU.

"If it's fair for other parliaments to have their say in that period, it's only fair for Scottish voters to have their say at the same time.

"What would be unfair is to deny people in Scotland their choice. That would be undemocratic and totally unacceptable."