Theresa May has already decided to block a second independence referendum if Nicola Sturgeon today pushes for a fresh vote on Scotland’s future, Whitehall sources have said.

UK Government insiders said the Prime Minister considered the issue had been “dealt with” decisively and would swiftly reject any effort to revisit the No result of 2014.

After several delays, Ms Sturgeon is finally due to update MSPs this afternoon on the timing of a second independence referendum and her response to the latest delay to Brexit.

Her spokesman said she would give a “detailed and substantive statement setting out a path forward for Scotland amid the ongoing Brexit confusion at Westminster”.

Ms Sturgeon will also “seek to strike an inclusive tone” in her 30-minute statement and an hour of questions from MSPs, he said.

The First Minister is expected to stress her determination to achieve independence through a referendum, while acknowledging the public has yet to be convinced about going through more constitutional change given the upheaval of Brexit.

Facing calls from an impatient Yes movement to use her mandate for Indyref2 before it expires in 2021, Ms Sturgeon is expected to call again for Westminster to transfer the power to hold another referendum to Holyrood under a so-called Section 30 order.

At Ms Sturgeon’s instigation, Holyrood voted 69-59 for a Section 30 order in March 2017, but Mrs May refused to grant it, sticking to her mantra of “now is not the time”.

Ms Sturgeon could cite a second refusal as more proof of Westminster ignoring Scotland’s wishes in the run up to the next Holyrood election.

But the slow pace could disappoint gung-ho independence supporters, including some of her own parliamentarians, who do not want to “dither” over the SNP’s founding goal.

Ms Sturgeon recently ruled out a Catalan-style wildcat vote as it would end in a legal quagmire, leaving her reliant on the UK Government granting a Section 30 order.

TOM GORDON: The complex dance between Brexit and Indyref2

One SNP MSP said that meant no referendum in this parliament.“She said she wouldn’t do it without a Section 30 order and she’s not going to get one. So it’s not happening.”

Another said: “I think there will be enough for the sensible people to be happy that we’re moving forward. Some of the others will never be happy, but it’s not about them.

“We need to convince the people that were too scared in 2014 to make the change. A lot of us have been treading this road a long time. A few more miles won’t make a difference.”

The Unionist parties at Holyrood said Ms Sturgeon should concentrate on health, education and the economy, not independence.

Fearing an anti-climax, the pro-independence Greens said it would be “hugely disappointing” if the First Minister failed to use the ‘triple-lock’ mandate she says she has to hold another vote.

Ms Sturgeon said in June 2017 that she would update MSPs a “precise timescale” for an independence vote by last autumn, when the terms of Brexit were expected to be clear.

However she repeatedly delayed an announcement because of Brexit uncertainty.

Titled “Brexit and Scotland’s Constitutional Future”, the statement comes just three days before the SNP spring conference in Edinburgh, when party activists are set for a heated debate on which currency would be used in an independent Scotland.

Although confusion over Brexit continues, making a statement today confirming her desire for a referendum potentially heads off a bigger row at conference.

The First Minister briefed the Scottish Cabinet on her plans yesterday, but will not tell her MSPs until this morning in order to minimise the chance of leaks.

Her spokesman said no documents or legislation would be published with the statement, suggesting it will be a rhetorical rather than a practical step towards Indyref2.

The Scottish Government consulted on a draft referendum bill more than two years ago, in the aftermath of the EU referendum vote, but little has been heard of it since.

Tory MSP Maurice Golden, whose party opposed changing the parliamentary timetable to allow today’s statement, said: “If Nicola Sturgeon wants to give a statement, not about schools, the economy or hospitals - but about a second independence referendum - then she is making her priorities absolutely clear. We want to move on from the SNP’s constitutional grandstanding, and get back to the things that matter to people.”

But in a sign of division in the Yes movement, Green MSP Patrick Harvie said: “The day after the Brexit vote the First Minister sounded like she was firing the starting gun on a referendum campaign but nearly three years on the race hasn’t started, despite Scotland having been subjected to utter contempt from Westminster all that time.

“There is a mandate to hold a second independence referendum within the current session of the Parliament and it would be hugely disappointing, as well as a dangerous precedent, to let that expire in the face of Tory obstructionism.

“Scotland needs an escape route from a Brexit it didn’t vote for and the Scottish Greens stand ready to campaign hard for an independent Scotland in the EU.”

Scottish Labour leader Richard Leonard said Ms Sturgeon had “no evidence” the people of Scotland wanted another independence referendum.

“Leaving the UK would lead to unprecedented austerity for Scotland’s public services. Each currency option the First Minister has tried simply makes that worse.”

Scottish LibDem leader Willie Rennie added: “The First Minister should take the opportunity to tell parliament that she is ruling out another independence referendum. The SNP’s relentless independence focus makes it look like they have their fingers in their ears.”

Pamela Nash, chief executive of the anti-independence group Scotland in Union, said: “Nicola Sturgeon should use this opportunity to drop her reckless threat of a divisive and unwanted second independence referendum.

“We know the SNP only cares about creating more division, but the majority of people in Scotland want the government to get back to the day job and fix the crises in our schools and hospitals. It’s time for the First Minister to end the damaging uncertainty and work to bring the country together.”