DAVID Cameron might well be back in Downing Street later today but it could be a bittersweet moment; the stratospheric rise of the SNP may mean he will be the last Prime Minister of the United Kingdom.

The future of the Union will once again take centre stage at Westminster in the wake of the predicted outcome of a Nationalist rout in Scotland. Alex Salmond made clear on the back of the predictions that David Cameron now had "no legitimacy" in Scotland.

As England looks to move to the Right, Scotland shifts to the Left. If the 300-year-old partnership is to survive, then something has to be done.

If the exit poll is right, then the SNP will become Britain's third party with a major presence on the green benches, more responsibility and millions more pounds in UK tax-payer support.

For many, David Cameron's victory was based on the "Ajockalypse Now!" fear factor of a Lab-SNP alliance amid claims that he played to English nationalism with, among other things, the proposal for Evel, English votes for English laws.

Tristram Hunt, the Shadow Education Secretary, accused the Prime Minister of "playing fast and loose" with the Union, putting party before country, and that the Tory Party had been peddling "ugly sentiments" towards Scotland, which could have damaging consequences.

Amid all the fall-out, one result looks like the removal of the last Conservative MP for Scotland. The Tories hoped that their championing of the Unionist cause in the independence referendum campaign would enhance their fortunes; it looks like it hasn't.

The cause for the Union will be made all the harder by the fact that a future Scottish Secretary of State in a Tory government will be from an English constituency albeit a Scot, say, Michael Gove or Michael Fallon.

The image of the Secretary of State being a visiting viceroy in a white suit and pith helmet will be played upon by the Conservatives' opponents. Whoever gets the role, will have his work cut out.

If it is a virtual clean sweep for the Nationalists, Mr Cameron's Unionism is going to be sorely tested in the new parliament.

Lord Strathclyde, the Scottish Tory peer and former Leader of the Lords, insisted saving the Union was "the number one priority" for the new government because of the rise of the SNP.

He argued the next UK administration would have to find new ways of "bringing the people of the United Kingdom together".

The interesting intervention by Boris Johnson the other day when he mentioned the f-word, federalism, possibly points to a new line in the sand for defenders of Unionism; that is, to stop Scotland splitting from the UK, a federal system might begin to be proposed by more politicians at Westminster as the only way forward.

Interestingly if the Tories do fall short of a majority and decide to do a deal with the Democratic Unionists, one of their demands is to have a commission on the future of the Union and how to strengthen it.

But it could all now be too late.

Nicola Sturgeon who will strut the Westminster stage this weekend will be determined to build on the momentum that the SNP is building; having won a landslide at Holyrood in 2011 and now predicted to win another at Westminster in 2015.

How impossible is it now for her not to include a commitment to hold a second referendum in Scotland in the SNP's 2016 Holyrood manifesto? The pressure from within her party will be immense and almost certainly irresistable.

She will probably wait until after the planned 2017 EU referendum to hold another poll but the trigger, the "material change", could come before that in the Commons vote to renew Trident. This is due, coincidentally, in early 2016; that is, in the run-up to the Scottish parliamentary elections.

A second referendum in spring 2018 might now be odds-on; Mr Cameron might flinch at having to fight the same battle all over again. But with the SNP surge, his chances of pulling off another victory look much worse.

His epitaph as the last Prime Minister of the United Kingdom could still be written.