Scottish parliament to vote on holding a second independence referendum in 2018/19.

Vote will put pressure on Theresa May to reverse decision to block a new vote.

Polls suggest 50/50 support for independence.

SNP believes Brexit could be a game-changer.

LONDON — The Scottish Parliament will on Wednesday vote on whether to call a second independence referendum before Britain leaves the EU.

So will Scotland leave the United Kingdom an if so, when? Here's everything you need to know.

When will there be a second Scottish independence referendum?

We don't yet know. The Scottish parliament are likely to vote in favour of a second referendum to be held at some point between Autumn 2018 and Spring 2019 when Britain is due to Brexit. However prime minister Theresa May has ruled out any new poll before Britain leaves the EU, saying that it would "not be fair" to the Scottish people to hold one before then.

Does Sturgeon have the power to call another referendum with May's blessing?

No. In the consultation for the Scottish government's draft referendum bill, they acknowledge that "a section 30 order would be sought and agreed" from the UK government.

What's a section 30 order?

Section 30 orders are a means of transferring powers over to the Scottish government that would normally be reserved by Westminster. The power to call a referendum is one of them.

So will Theresa May hand over those powers?

We don't know but certainly not yet. Theresa May said last week that it would "not be fair" to the Scottish people to hold a second referendum before the terms of Britain's Brexit deal is in place. She also insisted that doing so would damage the UK's negotiating position with the EU. However, she has so far neither ruled out or ruled in holding a referendum once Britain has left the EU.

Might Sturgeon hold a referendum before Brexit anyway?

It's possible. She could argue that if a majority Scottish Parliament vote in favour of a second referendum then it would give her the mandate to hold one, regardless of whether such a vote would be legally binding. If such a vote were held and a majority of Scottish people voted to leave, then it would be politically difficult for the UK government to ignore the vote. However, Sturgeon currently appears reluctant to go down this route and is still applying pressure for for May to agree a new poll.

I thought Scotland already voted to stay?

Correct. The first Scottish independence referendum was held in 2014 and was won by a margin of 55% to 45% for the No side.

And didn't the SNP say the referendum would settle matters?

They did. In the run up to the last referendum result the then Scottish First minister Alex Salmond said: "this is a once in a generation opportunity, perhaps even a once in a lifetime, opportunity for Scotland."

He also suggested that the next referendum probably wouldn't happen for another 18 years. "If you remember the previous constitutional referendum in Scotland [on devolution], there was one in 1979 and then the next one was in 1997. That's what I mean by a generation," he said.

So what's changed?

Two things. The EU referendum result last year means Scotland now faces being forced out of the EU despite a clear majority of Scots voting to Remain. The second factor is Theresa May's decision to opt for a "hard Brexit" and leave the single market and the customs union. In their manifesto last May, the SNP promised that "the Scottish Parliament should have the right to hold another referendum... if there is a significant and material change in the circumstances that prevailed in 2014, such as Scotland being taken out of the EU against our will."

Sturgeon now believes that we have reached such a material change.

Have the Scottish people changed their minds?

The evidence is mixed.

Recent polls have found that Scots are roughly split 50/50 on whether there should be another referendum within the coming years, with a very slight majority now saying there should. However, there is some evidence that Brexit could have an impact on Scots' views. When pollsters Panelbase told respondents earlier this year that it would no longer be possible to be part of both part of the UK and the EU, more said they would like to leave the UK than stay in it.

So there's a majority in favour of leaving the UK now?

No. When asked the simple question "Should Scotland be an independent country?" most polls suggest the Scottish people are still in favour of staying. In fact, the polls now are in an almost identical position to where they were before the last independence referendum. However, one most recent poll, by Ipsos Mori did find a one-point majority to leave.

Here are all the most recent polls in full

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First Minister of Scotland and leader of the SNP Nicola Sturgeon, votes with her husband Peter Murrell on May 7, 2015 in Glasgow, Scotland. Photo by Jeff J Mitchell/Getty Images

Will Scotland ultimately vote to leave this time around?

At the moment the polling remains largely unchanged from when the last referendum was held. The hopes among some in the SNP that the Brexit vote would cause a surge in support for independence have not yet been realised. That may change, of course, once negotiations with the EU begin and the realities of Brexit become clear.

Theresa May's decision to block a referendum before Brexit may also create a longer term resentment towards Westminster and the Union.

However, the same economic arguments that largely won the independence referendum for the No side would still apply and indeed have arguably been heightened by Brexit. The latest statistics show that the value of trade between Scotland and the rest of the UK is worth four times that of the value of trade between Scotland and the rest of the EU. Far from propelling Scotland out of the UK, Brexit could actually reinforce existing fears of Scots about leaving the safety of the Union.