Britain on brink of political upheaval with SNP surge set to redraw Scotland's political map

With this in mind, most pundits believe the key question is whether or not David Cameron can win more than 290 seats to make the Tories the largest party and in a position to gain power with the help of a diminished group of Liberal Democrat MPs and, possibly, a further deal with Ulster's Democratic Unionists.

But if the Conservatives cannot achieve the magic number - estimated to be 323 when the abstentionist Sinn Fein MPs are excluded - then Ed Miliband will, by law, get the chance to form a government with the help of others, including the Lib Dems.

Despite questions being raised of late about what would make a government legitimate, Lord O'Donnell, the former Cabinet Secretary who oversaw the formation of the 2010 Lib-Con Coalition, said constitutional convention was clear.

"Remember, we live in a parliamentary democracy...When Scotland voted to stay in the Union in its independence referendum, they knew what the rules of the game were, as it were, and they are very clear.

"They are laid out in the Cabinet Manual and it says that the ability of government to command the confidence of the elected House of Commons is central to its authority to govern," he explained.

If tomorrow's result does produce a hung parliament, then inter-party talks will begin immediately tomorrow with negotiating teams set up at Westminster. Coincidentally, Nicola Sturgeon will be in London to represent the Scottish Government as part of the VE Day commemoration.

Despite the fact the Labour leader has categorically ruled out any formal deal with the Scottish Nationalists, it might fall to the SNP leader and her expanded team of MPs to help Mr Miliband gain power by voting for his party's Queen's Speech and Budget.

The remarkable rise of the Scottish Nationalists in the eight months since the independence referendum is expected to sweep away the old order, resulting in defeats for a number of senior Scottish political figures.

These could include: Danny Alexander, Chief Secretary to the Treasury; his Lib Dem colleague Jo Swinson, the Business Minister; as well as Labour's Douglas Alexander, the Shadow Foreign Secretary; Margaret Curran, the Shadow Scottish Secretary, and the Tories' sole Scottish MP in the last parliament, David Mundell, the Scotland Office Minister.

A poll of polls by the psephological website What Scotland Thinks has pointed to the SNP winning 55 out of 59 Scottish seats, up from six in 2010, with Labour gaining just three, down 38 since five years ago, and the Liberal Democrats picking up one, a fall of 10.

This would also mean, in Westminster terms, Scotland would once again become a Tory-free zone.

The final YouGov poll on the overall result, published last night, placed the Conservatives and Labour neck and neck, at 34 per cent each. The pollster's analysis of the survey of more than 10,000 voters suggested the Tories are on course to win 284 seats versus 263 for Labour.

With 1,351 voters surveyed in Scotland, it had the SNP securing 48 per cent of the votes. The party won just 20 per cent in 2010. YouGov's own analysis suggested the nationalists' share of the vote would leave them with 48 seats.

One consequence which could result from a leap in the number of Nationalist MPs is a renewed push for a second independence referendum. Pressure is expected to mount on Ms Sturgeon to include a commitment to this in her party's 2016 Holyrood manifesto within weeks if not days of tomorrow's watershed result.

Another consequence will be the raising of questions about the future of the United Kingdom.

Boris Johnson, the London Mayor, and potentially the next Conservative leader, has acknowledged the fall-out to the election could have major constitutional ramifications, saying: "After the election we need to have a sit-down and people need to think about the future of Scotland.

"I want a United Kingdom but we may have to think about a federal structure for the UK. I'm on for that."

At the end of 36 hours' campaigning, which saw David Cameron travel from Cornwall to Wales and from Scotland to Cumbria, the Prime Minister appealed to voters for more time to build a better Britain as he insisted the election would "define a generation".

In a final rallying cry in the marginal seat of Carlisle, he said: "It comes down to a choice of leadership; whether you want me to continue leading our country and taking it forward or whether you want Ed Miliband to go back to the start and waste all the work of the last five years."

Watched by wife Samantha, the Tory leader said if he was returned to power, he would keep growing the economy, getting people back into work and boosting childcare.

By contrast, he warned Mr Miliband could only govern if he was "held to ransom" by Ms Sturgeon and the SNP. "I don't raise this as some irrational fear," declared Mr Cameron. "I raise it because people are raising it with me day after day."

Mr Miliband, who after touring marginals in Lancashire, ended his campaign in Leeds, issued a special message to Scotland, saying: "If I'm Prime Minister, I will hold Scotland's interests in my heart and my head. A Labour Government is within touching distance. I'm asking for your vote to kick the Tories out and to end Tory austerity. Vote for Labour; the party of Scotland, of change and of social justice."

Insisting this was the closest election for a generation, the Labour leader told the Leeds rally that he did not want Britons to wake up tomorrow and find they were living in "a Tory decade".

He added: "I'm not asking you simply to vote Labour; I'm asking you to vote to reward hard work again, to build a future for all our young people and to rescue our NHS."

Ms Sturgeon, who campaigned in Edinburgh, promised that if Scottish voters "put their trust in the SNP today, we will not let them down".

She added: "This election offers an opportunity to make Scotland stronger and stand up for progressive politics, which we simply can't afford to pass up and people should grab it with both hands as they enter the polling stations today."

Nick Clegg, who ended his final push for votes at Thurso after visiting Glasgow, East Dunbartonshire and Inverness, claimed the Liberal Democrats would be the "surprise story" of election night, doing better than pollsters had predicted.

He earlier accused the Conservatives of no longer being a party for the whole UK but had "basically mutated into an English party chasing Ukip votes in southern England".

The eleventh-hour political messages came as reports suggested unnamed union bosses - with the prospect of a hung parliament widely predicted - were urging Mr Miliband to make the Lib Dems an immediate offer of electoral reform to get Mr Clegg's party to choose Labour rather than the Tories as coalition partners.

But Chuka Umunna, the Shadow Business Secretary, branded the reports as "rubbish" and insisted there were "no pacts, no deals, nothing".

Amid the welter of opinion polls, one, an online survey of 1000 people by uSurv, took the temperature on what voters felt about the impact of the election campaign.

A majority, 56 per cent, admitted that all the parties' campaigning of the last six weeks had had no effect whatsoever on the way they would vote when polls open at 7am today.