A deal between the SNP and Labour after the next election would jeopardise Britain’s security for a “childlike world view” more suited to a “student protest group than government”, Defence Secretary Michael Fallon will warn.

Mr Fallon will announce that the Conservatives will include a pledge in their manifesto to build a new fleet of four Trident submarines to replace the existing system and provide a continuous nuclear deterrent.

And he will warn that a potential tie-up between Labour and the SNP after 7 May could result in Trident either being scrapped on scaled back in a way that would “dangerously weaken our collective defence”.

Labour has committed itself to replacing Trident if it wins the election but has said it would look at reducing the number of nuclear submarines from four to three. By promising to build the full fleet the Tories hope to create a dividing line with Labour on an issue which has always played well for the Conservatives.

Britain's Defence Secretary Michael Fallon (Getty Images)

In a speech, Mr Fallon will warn that Mr Miliband is prepared to barter away Britain’s nuclear deterrent to buy support from the SNP leader Nicola Sturgeon.

“The SNP’s childlike world view would sacrifice the long-term security of the UK and play into the hands of our enemies,” Mr Fallon will say. “The nuclear deterrent protects all of Britain and the SNP represents a separatist threat that would dangerously weaken our collective defence.

In pictures: Experts' predictions for the General Election - 04/04/15 Show all 10 1 /10 In pictures: Experts' predictions for the General Election - 04/04/15 In pictures: Experts' predictions for the General Election - 04/04/15 Andrew Hawkins (ComRes) “My position has moved: no party can win a majority now. I have also shifted in favour of the Conservatives winning more seats than Labour. That, however, assumes that the current Tory momentum is maintained and that they don’t do anything daft or careless between now and polling day. But the underlying pattern is distinctly in their favour.” (In January he predicted Labour would be the largest party, possibly with a small majority.) Andrew Hawkins In pictures: Experts' predictions for the General Election - 04/04/15 Joe Twyman (YouGov) “Probably: a ‘well hung parliament’. Possibly: Conservatives winning most votes and seats, thanks, in part, to SNP gains at Labour’s expense. Speculation: Conservatives unable to form another coalition, not having enough seats with just the Lib Dems, but Labour better placed with SNP and Lib Dems – albeit informally.” (In January Twyman said: “Gun to my head? Labour minority government.”) In pictures: Experts' predictions for the General Election - 04/04/15 Ben Page (Ipsos MORI) “Stuck in ‘too close to call’ mode still, made harder by the way votes translate into seats in Parliament. If the parties remain neck and neck, Labour might just end up with more seats, but not a majority. We still have weeks of campaign to go and no clear picture for the marginals, where the polling that is being done suggests a lot of local variations that have plenty of potential to surprise us in May.” (Last time Page said it was a “mug’s game” to make predictions four months before an election.) In pictures: Experts' predictions for the General Election - 04/04/15 Rick Nye (Populus) “Since January, the Conservatives have clearly improved on the polls relative to Labour to the point where I’d expect the Conservatives to win the most seats as well as the most votes 7 May. What’s less clear is whether the Conservatives would be able to form a government. (In January Nye expected a hung parliament in which Labour would win most seats but not necessarily most votes.) In pictures: Experts' predictions for the General Election - 04/04/15 Nick Moon (GfK) “Something would need to change dramatically for there to be any chance of a one-party majority government. My guess: the Tories will be largest party, but some way short of forming even a two-party coalition. A Labour minority government seems most likely, but I won’t be putting money on it.” (Prediction unchanged since January.) In pictures: Experts' predictions for the General Election - 04/04/15 Damian Lyons Lowe (Survation) “On Survation’s public polling, Ed Miliband remains the person most likely to form the next government. However, he’s far from the workable majority figure required [321]. Friday 8 May will remain a day of deals and discussions with other parties to form the next government.” (In January he expected Labour to be the largest party in a hung parliament, by 40-50 seats over the Conservatives.) In pictures: Experts' predictions for the General Election - 04/04/15 Michelle Harrison (TNS) “It’s less a case of who wins but who can scrape over the line. Labour polls better on the NHS; the Tories poll better on the economy. Can any claim additional territory from the other over the remaining weeks? Probably not. But our polls show that the public thinks the Tories will be the largest party. In the absence of a firm lead, I’ll go with the wisdom of crowds.” In pictures: Experts' predictions for the General Election - 04/04/15 James Endersby (Opinium Research) “Despite the recent weekly statistical ties, we’ve witnessed the faint whispers of movement in the air and a slow, unsteady and shaky sway towards the Tories. How this shift plays out over the coming weeks obviously depends on a huge number of factors. My call, if this holds fast and momentum gathers: Conservatives 288, Labour 267, SNP 45, Lib Dems 24, Plaid Cymru 3, Ukip 3, Greens 2.” (In January he put Labour on 320 seats.) In pictures: Experts' predictions for the General Election - 04/04/15 Martin Boon (ICM) “I’m tempted to say: how should I know? I’m just a pollster. But I feel that Miliband may just have raised himself from the grave, so I’ll add a couple to where I had them before. Everyone else largely becalmed although, I see the Greens disappearing from view and Ukip sliding a touch. Tories 34 per cent, Lab 32 per cent, Lib Dems 14 per cent, Ukip 12 per cent. I don’t trust any academic model that translates vote shares into seats, so a seat projection from this is a pure and simple guess, which is Labour to be touching 300 seats with the Tories just behind.” (In January Boon predicted Labour on 290 seats.) In pictures: Experts' predictions for the General Election - 04/04/15 Lord Ashcroft (Lord Ashcroft Polls) He refuses to make predictions. “My polls are snapshots, not predictions.” Rex

“When Britain faces nuclear blackmail by rogue states, this self-indulgent approach is more suited to a student protest group than a party of government.”

The announcement comes as a new poll found that only 15 per cent of Tory activists expect their party to win a majority.

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