It appears that Nicola Sturgeon has become the most dangerous woman in Britain. In an interview with the Guardian the SNP leader said her party would have a “huge ability to change the direction” of the government in the event of a hung parliament.

And Sturgeon’s deputy, Stewart Hosie, gave a giant clue as to what that might mean. He said during a TV interview that the SNP could vote against “any bit of spending” that it didn’t agree with, such as Trident.

Together, those statements outraged both the prime minister, David Cameron, and the editors who support his return to Downing Street while opposing the possibility of an accord between Ed Miliband’s Labour and the SNP.

Gone is the praise for her TV performances. Now she is the election’s demon figure as far as Fleet Street’s blue newspapers are concerned.

The SNP will hold Britain’s defence to ransom, says the Times. The SNP is prepared to paralyse the armed forces, says the Daily Telegraph. She will hold UK defence to ransom, says the Times. The SNP is threatening to hold the UK to ransom, says a Daily Mail headline across two pages. The i, which does not toe the Tory line, carries the splash headline, “SNP veto on Labour spending”.

The sudden realisation that the Scottish nationalist tail could end up wagging the Westminster (Labour) dog is the major concern of most of today’s London-based national newspapers.

“The SNP plans to co-operate with Labour enough to keep it in power but meddle in unprecedented ways with individual budget items”, says the Times’s editorial. It continues:

“The uncertainty on policy that this will bring will be as nothing next to the constitutional chaos caused by the SNP’s refusal yesterday to rule out another referendum on Scottish independence... The SNP is a separatist party on the cusp of national power, and is not afraid to say how it would use it. It would back Labour on cutting tuition fees, scrapping the coalition’s welfare reforms and fixing energy prices. The signs are that at the same time it would hold the UK budget to ransom with line-item deliberations... This is not a basis for stable government”.

The Times concludes that Sturgeon’s offer to Miliband to help him to “end the Tory agenda” is only possible “by ignoring every basic rule of fiscal prudence” and “worse still would be her price — her own agenda — which is the break-up of Great Britain”.

Making a similar point, the Telegraph’s leading article fears “a party that is literally anti-British... will effectively hold the whip hand in a Westminster parliament... The SNP, which may win around one million votes in an electorate of more than 40 million, will have power to dictate policy in the rest of the UK where it is not represented”.

It argues that this amounts to “the antithesis of democracy” and concludes:

“The flaws in Labour’s devolution strategy, which was meant to slay the dragon of independence not give it new life, have been exposed for all to see. Once again, the threat to the Union is real”.

The Mail is withering about the SNP’s leader. “Even by her own megalomaniac standards, Nicola Sturgeon’s language on the day of her manifesto launch is breathtaking in its arrogance”. (It does not say this in its Scottish edition, which carries a different, though avowedly unionist, editorial).

The Mail believes the SNP, a party representing less than 5% of the total electorate, is “intent on destroying the UK” and that the prospect of its ruling in company with Labour “should make anyone who believes in democracy shudder”.

The Sun refers to Sturgeon as an arrogant leader whose chief motive is to ‘lock the Tories out’ of power” regardless of whether most voters in England want the Tories in government. It continues:

“Labour’s agenda — already the most left-wing in 30 years and an imminent danger to the recovery — would be dragged even further to the left. Miliband would drive the economy back to the cliff edge. The SNP — bent on breaking up the UK — will bind him hand and foot, and floor the accelerator... If Cameron doesn’t get a majority this is our bleak future”.

The Daily Express’s columnist, Leo McKinstry, think it would be an “undemocratic farce” and “a disastrous scenario” should the SNP hold the balance of power in the next parliament. He warns that Sturgeon, “brimming with tartan ideological fervour” is aiming to form with Labour “a Caledonian socialist pact” and “an anti-English pact”.

The non-Tory Independent also sees problems ahead:

“The greatest of many headaches the Labour chief whip will have in the next parliament is how to prevent left-wing Labour MPs from being seduced by SNP promises on public spending and Trident, and from voting against their own minority Labour government’s budget”.

The Financial Times, while conceding that the SNP opposes “the existence of the UK as we know it” and “might behave provocatively [by] tugging a weak Labour government to the left” thinks this “cause for concern, but not panic”. It argues:

“Britain has not suddenly lost its ancestral gift for improvisation. It has a way of finding a path through political and constitutional problems... The system depends on opposition parties resisting the temptation to bring down the government. Look closely and they have an incentive to show such forbearance. If the SNP win virtually every seat in Scotland, as polls suggest, they can only stand to lose if there is another election soon after. They will not want to take risks before the Scottish elections in 2016”.

The FT thinks in the “haggling culture of coalition” the prime minister of the day “might proceed on a bill-by-bill basis or seek a “confidence and supply” agreement.

It sees a virtue in the Fixed Term Parliaments Act because it “prevents an election for five years unless a supermajority in the house of commons votes for one, or the government loses a confidence motion and no other administration is formed within 14 days. This provides some insurance against the uncertainty of another election... Do not underestimate the British knack for muddling through”.