THERESA May has “sealed the fate of the union” by denying Scots a second referendum, Nicola Sturgeon claimed last night, as the UK’s most powerful politicians became locked in a stand-off that threatens to spiral into a full-blown constitutional crisis.

The First Minister even offered to go head to head with Mrs May in a televised debate after she ruled out a referendum until at least after Brexit in 2019.

In a day of fast-moving events on both sides of the border, the Prime Minister said “now is not the time” for a fresh vote on independence given the two years of complex Brexit negotiations that start next month.

Mrs May said she was on the “right side of the Scottish people” in rejecting one on the SNP's preferred timetable.

The First Minister said on Monday that she wanted to go to the country again between autumn 2018 and the spring of 2019, so that voters could choose between a hard Brexit in the UK and independence and a closer relationship with Europe.

However Whitehall sources pointed to polling evidence, which showed that most Scots simply did not want another divisive poll.

As the constitutional battle lines were drawn, Mrs May followed Ms Sturgeon’s example of pre-empting the triggering of Article 50 by pre-empting the SNP leader’s speech to her party’s spring conference with her own dramatic announcement.

In the wake of an acrimonious First Minister’s Questions at Holyrood, the Prime Minister told ITV News: "Right now we should be working together, not pulling apart.

"We should be working together to get that right deal for Scotland, that right deal for the UK, as I say that’s my job as Prime Minister and so for that reason I say to the SNP: now is not the time.”

She added: "More than that, it wouldn’t be fair to the people of Scotland because they’d be asked to make a crucial decision without the necessary information, without knowing what the future partnership will be or what the alternative for an independent Scotland would look like.”

Later, at a news conference in Edinburgh, the Scottish Secretary David Mundell confirmed the UK Government would “not be entering into discussions or negotiations about a Section 30 agreement and any request at this time will be declined”.

This means that next Wednesday when MSPs are expected to vote to mandate Ms Sturgeon to seek authority to hold a second referendum, the Prime Minister will refuse the request.

Speaking alongside Mr Mundell, Scottish Conservative leader Ruth Davidson said: “We reject conclusively the timetable for a referendum set out by the Scottish Government for a key reason; because it is unfair to Scottish voters.”

And she added: “We’ve just come through a referendum campaign, where a key complaint among many people was that they did not have the necessary information to help them make an informed decision.

“If we were to keep to the First Minister’s timetable this is exactly what would happen in Scotland too. On the most important political decision a country can make we would be voting blind.”

But Ms Sturgeon responded that it would be a “democratic outrage” to deny the plebiscite.

She said: “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.

“It is for the Scottish Parliament - not Downing Street - to determine the timing of a referendum and the decision of the Scottish Parliament must be respected,” she declared.

The First Minister added that it would be “outrageous” for the Scottish Parliament to be “frozen out of the process”.

“The Scottish Government has a cast-iron democratic mandate to offer people a choice and that mandate must be fulfilled,” she said.

Later in a BBC Scotland interview, she added Mrs May’s attempt to block the democratic will of the Scottish Parliament “seems to me to be going back to the bad old days of Margaret Thatcher”.

Ms Sturgeon described Mrs May’s rejection as an “argument for independence really in a nutshell”; that Westminster believed it had the right to block the democratically elected mandate of the Scottish Government and the majority in the Scottish Parliament.

“History may look back on today and see it as the day the fate of the Union was sealed,” she added.

However, the First Minister’s office has appeared to accept a second referendum might not take place until May 2021.

At a media briefing after the Theresa May’s statement, Ms Sturgeon’s spokesman was asked the cut-off date for another poll and if the Scottish Government might accept a referendum up to 2021.

He replied: “The First Minister has made clear her preferred timescale and that is the timescale we are working to. But the mandate is for this parliament. The basis on which this government was elected is the mandate in the manifesto; the manifesto covers the parliamentary term.”

Downing Street’s rejection of the referendum demand will set the scene for a heighten atmosphere at the SNP’s two-day conference in Aberdeen, where Ms Sturgeon is due to address the Nationalist faithful today.

At Westminster, a senior Whitehall source said the PM would “not compromise” on anything that would get in the way of securing the best Brexit deal for Britain and made clear the UK Government had no intention to engage in further negotiations with the Scottish Government on another timeframe.

Meanwhile, the Prime Minister’s spokesman explained the timing of her seismic interview, saying: “The Prime Minister made her announcement today so the Scottish Parliament was informed of our position when they vote [to request power to hold a referendum]”. He added: “We still hope they will step back from that vote.”

Today, in a speech at the Conservatives’ spring conference in Cardiff, Mrs May will unveil her vision for the country, a Plan for Britain, which will put strengthening and sustaining the “precious, precious Union” at its heart.