Fears that the next government will be hamstrung by claims that it lacks democratic legitimacy have been intensified by new research for The Independent suggesting that seven out of 10 people believe the Scottish National Party (SNP) should not be able to veto any UK government policies if they do not affect Scotland.

There is strong opposition in England and Wales to the SNP enjoying huge influence in a hung parliament, the survey of more than 2,000 people by ORB suggests.

It follows repeated claims by the Conservatives that Nicola Sturgeon’s party would call the shots in a minority Labour government.

With opinion polls suggesting a deadlock between the Tories and Labour, the Liberal Democrat leader, Nick Clegg, has predicted a second election before Christmas if either of the two main parties attempt to “stagger through with a messy and unstable minority government”. Ed Miliband is adamant he would do “no deals” with the SNP if Labour fails to win a majority in the election – but could still rely on its informal support to keep a Labour government in power. However, according to The Independent’s poll, most Britons would not accept the SNP wielding influence over English policy.

Only 29 per cent agree that the SNP should enjoy a veto over government policies even if they do not affect Scotland, while 71 per cent disagree. A majority of people in every part of Great Britain except Scotland oppose a veto.

But in Scotland people back the idea of the SNP enjoying such influence by 63 per cent to 37 per cent.

Sturgeon inside No 10: the SNP leader poses in a playhouse as she visits ABC Nursery in Dedridge, Scotland, while on the election campaign trail (Getty) (Getty Images)

On the last day of campaigning, the Labour leader will pledge that any government he leads would abolish “non-dom” status, under which some residents in Britain with links to another country do not pay taxes on their overseas income.

He will say: “Anyone permanently resident in the UK will pay tax in the same way. And only Labour will do this. We have come to expect David Cameron and Nigel Farage defending the richest and most powerful. But it is extraordinary that Nick Clegg is defending the non-dom rule too. This is the choice at the election: a Labour government that will put working people first or a government that will stand up only for a privileged few.”

Mr Clegg warned that a Tory minority government would collapse if it failed to offer “sweeteners” to the SNP, Ukip or Northern Ireland’s Democratic Unionist Party, while a minority Labour administration could not pass a Queen’s Speech without giving Scotland full fiscal autonomy to buy off the SNP.

VIDEO: WHAT MIGHT HAPPEN AFTER MAY 7?

The Deputy Prime Minister said: “Everybody knows that no one will win this election – even if David Cameron and Ed Miliband won’t admit it publicly.

“The Lib Dems have shown that coalitions can be strong and stable. But instead of creating stability, Labour and the Conservatives will create a shambles. If they try to stagger through with a messy and unstable minority government instead of putting the country first then they will risk all the hard work and sacrifices people have made over the last five years.

“The last thing Britain needs is a second election before Christmas. But that is exactly what will happen if Ed Miliband and David Cameron put their own political interest ahead of the national interest. The only party that will ensure stability is the Lib Dems.”

The Tories hope the spectre of a Labour government propped up by the SNP will provide the last-minute surge they have been seeking as people head to the polling booths.

In pictures: Experts' predictions for the General Election - 03/05/15 Show all 10 1 /10 In pictures: Experts' predictions for the General Election - 03/05/15 In pictures: Experts' predictions for the General Election - 03/05/15 Andrew Hawkins (ComRes) “The sclerotic, negative and risk-averse campaigns from the two main parties make it hard to see how much can alter. So, my prediction is the same – Tories get most votes, but Labour better placed to form a government. Then a long spell of political and perhaps constitutional chaos.” Andrew Hawkins In pictures: Experts' predictions for the General Election - 03/05/15 Joe Twyman (YouGov) “‘The world is changed, I feel it in the water, I feel it in the earth. I smell it in the air.’ So begins the film version of Lord of the Rings. – which is, of course, the famous tale of an epic journey culminating in the final battle between good and evil. The world of British politics has certainly changed. “With a few days still to go I expect that more change could still occur, but it is likely to be minor and the national level and more concentrated on the ground in the key marginal constituencies where the Hold Your Nose or Cut It Off to Spite Your Face™ message pushes home. I expect the Conservatives to be the beneficiaries, but it will not be anything like enough to make a difference to the overall result.” In pictures: Experts' predictions for the General Election - 03/05/15 Ben Page (Ipsos MORI) “As the only pollster to correctly predict a hung parliament last time – and then foolishly change my prediction when I saw ALL the others were saying a Conservative majority – I am going to say hung parliament again. With more Conservative than Labour seats. The SNP won’t wipe out the Labour Party completely in Scotland but will get them down to single figures. The Lib Dems will out perform their poll numbers and should get circa 26 seats – or more. Ukip will be delighted with four seats at most, probably fewer.” In pictures: Experts' predictions for the General Election - 03/05/15 Rick Nye (Populus) “Tories largest party, comfortably.” In pictures: Experts' predictions for the General Election - 03/05/15 Nick Moon (GfK) “SNP now 50, Ukip 2; Tories to be largest party in votes and seats, but still a Labour minority government.” In pictures: Experts' predictions for the General Election - 03/05/15 Damian Lyons Lowe (Survation) “Conservatives – I’m upgrading my seats prediction to 270-280 from 260-280. Labour – downgrading again to 265-275, based on the SNPs’ continued surge and Conservatives doing better in our seat-voting question as the election draws near and views are localised: SNP 45; Lib Dems 30; Ukip 6; Green 1; Respect 1. Ed Miliband will be the next prime minister.” In pictures: Experts' predictions for the General Election - 03/05/15 Michelle Harrison (TNS) “We enter the last few days of this campaign pretty much where we started. This election represents what happens when a country is not confident about its economic future, unsure of its place in the world, and fed up with the state of its politics. “The political stalemate at the centre, and the fragmentation of the traditional party system, has left us with a set of polls incapable of telling what will ultimately happen, when there are so many potential scenarios. What we can feel confident about though is that Thursday will be a seismic night for politics in Scotland. When the votes are counted, we expect the Tories to be the largest party, but that Labour should still have the greatest chance of forming a government. But how do we measure the advantage for the Conservatives of already being in No 10 in the days after the general election? The real drama will start on Friday.” In pictures: Experts' predictions for the General Election - 03/05/15 James Endersby (Opinium Research) “We saw some movement to the Tories, but the two big parties are back to being neck and neck with the Conservatives a hair’s breadth ahead. How this translates into seats or a coalition is unclear but based on our numbers we’d put the Conservatives ahead of Labour on vote share but the two parties within 10 seats of each other in the new House of Commons. The maths here gives Ed Miliband more options than David Cameron, so it might be sensible for voters to look up Ramsay MacDonald when trying to make sense of the result!” In pictures: Experts' predictions for the General Election - 03/05/15 Martin Boon (ICM) “The Tories appear to have developed a little momentum, which may or may not make any difference. I sense the now traditional herding of pollsters has begun, and the polls will coalesce around a Tory lead of between two and six points. I’ll guess at 36 per cent for the Tories and 32 per cent for Labour. The fight for third place could go either way. Beyond that I just don’t know what will happen and defer to the academics and gamblers when it comes to seat projections, and indeed when it comes to who on earth is going to form our next government. I’d like to apologise to Independent on Sunday readers for fence-sitting, but as I’ve said repeatedly of late: How should I know? I’m only a pollster.” In pictures: Experts' predictions for the General Election - 03/05/15 Lord Ashcroft (Lord Ashcroft Polls) He refuses to make predictions. “My polls are snapshots, not predictions.” Rex

Mr Cameron said that Mr Miliband would have a “massive credibility problem” if he tries to become prime minister without Labour winning the most seats. But Labour insiders believe that would be legitimate because what matters is securing a Commons majority – and the Tories could fail to muster one even if they win more seats than Labour.

As he completes 36 hours of non-stop campaigning, Mr Cameron will warn people that a vote for Ukip or the Lib Dems would open the door to an SNP-backed Miliband government that would “cost you money” as it would mean “heavy borrowing and higher taxes for more welfare”.

Gordon Brown, the former Prime Minister, acknowledged for the first time that the SNP could sweep the board by winning all of Scotland’s 59 seats – 41 of which are being defended by Labour. At a rally in Glasgow, he warned that such an unprecedented block of nationalist MPs would result in the Conservatives remaining in power.

Mr Brown, who is standing down after 32 years at Westminster, cited an SNP campaign leaflet which they would “demand more money for the NHS.” He insisted: “Voting Labour will mean we will deliver more money for the NHS in Scotland.”

Former Prime Minister Gordon Brown listens as Jim Murphy delivers a speech in Glasgow (Getty) (Getty Images)

With a third of Scottish voters undecided, Mr Brown said Labour could still avoid a wipe-out. He said social justice was in Labour’s DNA, but “the SNP can never bring themselves to accept that sharing [inside the UK] is the means to secure social justice, because nationalism is the imperative they serve.” He added: “Nationalism for the SNP dictates their every decision… and that is why there can be no deal, no tie-in, no arrangement, no compromise.”

Appearing with Mr Brown, Jim Murphy, Labour’s leader in Scotland, appeared to drop his guard and depart from the party’s “no deals” line. He told journalists: “This thing about co-operation with the SNP – we’ll deal with that after the election.” When pressed, Mr Murphy rowed back, saying he wanted to clarify the position. “There will be no deal, no formal coalition with the SNP,” he added.

What happens next? The final choices

Q | When will we have a clear picture?

A | If there is a tight-run race, it might take until the final results are declared on Friday afternoon to know whether David Cameron or Ed Miliband has the upper hand in forming the next government.

Q | What if Cameron has most seats?

A | He will have the first shot at attempting to stitch together a governing coalition – or could opt for a confidence and supply deal (where a partner would promise to support the Tories in Budget and confidence votes). His chances depend entirely on the numbers: if the Tories receive 290-300 seats they can probably get past the finishing line with Lib Dem and Democratic Unionist Party backing. Below that number he will face an uphill struggle.

Q | What if the Tories are narrowly behind Labour?

A | Because he is the incumbent Prime Minister, Cameron could try to build enough support for another term in office. But the arithmetic would be against him. And his party’s second place would raise questions about the legitimacy of the move.

Q | What if he cannot assemble enough support?

A | If the Tories are clearly ahead of Labour, Cameron could still declare victory and challenge Ed Miliband and the SNP to join forces to vote him out of office over a Queen’s Speech programme.

Q | Can he do that?

A | A constitutional grey area. Labour argues that if Cameron cannot command the confidence of the Commons, then Miliband should be given the opportunity to form a government and not have to wait until a minority Tory administration is defeated on a Queen’s Speech.

Q | What if Miliband has the most seats?

A | If Labour is clearly ahead, Cameron would be expected to vacate Downing Street, leaving Mr Miliband with the task of assembling a viable administration. Like Cameron, he could try to turn to the Lib Dems if his party picks up around 290 seats.

However, Labour is unlikely to win that many, leaving its leader with the headache of the SNP holding his fate in its hands. Miliband has promised there will be no deals (including a confidence and supply arrangement) with Nicola Sturgeon.

Q | Could Miliband then try to go it alone?

A | Yes, if there was no prospect of a stable Tory-led administration. Miliband could submit a Queen’s Speech and challenge the SNP to defeat it and potentially open the door to the Conservatives – or a second general election.