Glasgow, United Kingdom - Prime Minister Boris Johnson rejected an official demand for a second referendum on Scottish independence in a formal letter on Tuesday.

Responding to a request from Scotland's first minister and leader of the pro-independence Scottish National Party (SNP), Nicola Sturgeon, to hold a vote later this year, Johnson said Scotland had seen a decade of "political stagnation", with schools, hospitals and jobs left behind "because of a campaign to separate the UK".

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Sturgeon described Johnson's continued rejection as "unsustainable", and said the "Scottish government will set out our response and next steps later this month when we will also ask the Scottish Parliament to again endorse Scotland's right to choose".

"It was inevitable, but it then poses the bigger questions of the longer set of endgames that we are into," Scottish political analyst Gerry Hassan told Al Jazeera. "And, the need for the independence movement and the SNP leadership to have open public debates and an acknowledgement of the situation that [Scotland] is in."

Buoyed by his success in last month's UK-wide general election which saw him win a majority in Parliament, Conservative leader Johnson rebuffed Sturgeon's demands as he now looks ahead to Britain's withdrawal from the European Union - otherwise known as Brexit - on January 31.

Sturgeon's SNP may have secured 48 out of 59 Scottish seats in December's election, but the devolved Scottish Parliament in Edinburgh, where the party also dominates, requires permission - known as a Section 30 order - from Westminster in order to hold a second referendum on Scottish statehood.

The first was held in September 2014 and saw Scots reject independence from the British state by 55-45 percent.

The SNP leader has already ruled out holding a Catalan-style referendum on independence, which saw the Spanish region hold an unsanctioned poll in 2017 against the wishes of the Spanish government.

"Nobody realistically is thinking of unofficial referendums or a unilateral declaration of independence," said Hassan. "We are already looking ahead to 2021 and how a Scottish mandate [for independence] can be secured."

Today I have written to Nicola Sturgeon. The Scottish people voted decisively to keep our United Kingdom together, a result which both the Scottish and UK Governments committed to respect.



Let's make 2020 a year of growth and opportunity for the whole of the UK 🇬🇧 pic.twitter.com/JjQp3X2J2n — Boris Johnson (@BorisJohnson) January 14, 2020

In 2021, the next elections to the Scottish Parliament will be held - and today hold more intrigue than ever. The SNP will be hoping to secure a majority or at least return a pro-independence majority combined with the independence-supporting Scottish Green Party.

But even that route to securing another plebiscite on Scottish sovereignty looks forlorn after the Conservative Party Scottish Secretary at Westminster, Alister Jack, said on Sunday that victory for the SNP in the Scottish elections would not give the party a mandate to hold a second independence poll.

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"The ball's in Sturgeon's court, and it leaves the SNP in quite a difficult position," said Simon Pia, a former Scottish Labour press adviser. "Now it will be a question of how much popular support Sturgeon can gather, because the interesting point is that [at the UK general election] she gathered 80 percent of the seats in Scotland on 45 percent of the vote," he told Al Jazeera.

Pia also said that, while polling on independence remains between 45 and 50 percent, and within touching distance of a majority, it has never shown the clear majority support required to win another vote.

On Saturday, 80,000 pro-independence supporters braved the wind and rain in Scotland's largest city, Glasgow, to show their support for Scottish statehood. But as Sturgeon weighs up her next move - which could include taking the SNP's independence demand to the courts - the governments of Scotland and the United Kingdom will remain at loggerheads.

"What is quite lucky for Johnson is that this issue has come so early in his premiership," said Pia. "And he's still got a fair wind behind him. The discontent and criticism of the [UK] government has not started building up. But I can understand Sturgeon's position - she wants to play a waiting game."

Follow Alasdair Soussi on Twitter: @AlasdairSoussi