David Cameron today issues a passionate call for households around the UK to fly the Scottish flag to show their backing for a No vote in Thursday’s historic referendum.

With just four days to go until the crunch vote – and with the polls neck and neck – the Prime Minister called on families across England, Wales and Northern Ireland, to put the Saltire in their windows in support of the Union.

Mr Cameron’s rallying cry came as a new poll published yesterday put the No vote eight points ahead. According to Survation, in a poll commissioned by the Union camp, the No vote is running at 54 per cent while Yes is on 46 per cent.

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Getting passionate: With just four days until the crunch vote, David Cameron is calling for people to show public support for the union

However, former Chancellor Alistair Darling, the leader of the pro-union campaign, reminded nervous supporters that more than 500,000 people who were eligible to cast their ballot had not yet made up their minds.

Mr Darling said: ‘The situation is fluid. If we vote to leave by one vote there is no going back. We will have bought a one-way ticket.’

His warning seemed prescient last night as Downing Street was shaken by an ICM poll for the Sunday Telegraph which put Yes on 54 per cent and No on 46 – though polling experts warned that the small survey size of 700 meant that it was vulnerable to a ‘sampling error’.

An Opinium poll for The Observer put No on 53 and Yes on 47.

In his interview, Mr Cameron said the vote was ‘going down to the wire’ and he hoped a final lovebombing from around the UK could persuade the undecideds to vote No.

Describing the contest as ‘nerve-racking’, he said: ‘There are lots of things [people could do] whether it is using social media, tweeting and blogging your support for the UK; whether it is phoning a friend, talking to relations or work colleagues who live in Scotland and have a vote.

He wants families across England, Wales and Northern Ireland to put the Saltire in their windows

‘Or whether it is more visual demonstrations – hoisting the Saltire over buildings, putting a flag in your window – there are any number of things people should do.’

However, as Mr Cameron was urging support for the union, a Mail On Sunday poll of voters south of the border was pointing to a growing backlash against the idea of granting extra powers to the Scots to persuade them to vote No.

The ‘devo-max’ cross-party deal, including scope for Scotland to set its own taxes, was hastily brokered by Gordon Brown last week after a poll put the Yes camp ahead. The new poll found that 43 per cent of English and Welsh think the move was a ‘panic measure’, while 61 per cent object to the fact that Scotland already receives the highest level of per-head public spending in the UK. Just 12 per cent think that Westminster should bail out an independent Scotland if it runs into financial trouble.

The knife-edge final days of the campaign have contributed to party turmoil for both Mr Cameron and Ed Miliband. One senior Tory MP confided to this newspaper that a ‘clear majority’ of Conservative MPs expected Mr Cameron to resign if the Scots vote for separation, and that his leadership could still be in trouble if the pro-union camp prevails on Thursday.

The MP, who holds an executive position on the party’s powerful backbench 1922 Committee, said the rushed agreement to give the Scots greater devolution even if they voted to stay had annoyed rank and file MPs.

He said: ‘More than 50 per cent of the Parliamentary party think the PM should go if he loses. But there is also an iceberg contingent who will be restless if we win, because we have promised too much – and let Gordon Brown lead the way.’ He explained: ‘A few MPs are very visibly complaining, but an awful lot more are grumbling out of sight.’

Mr Miliband is facing internal ructions over Mr Brown’s promise of enhanced devolution powers, with Shadow Chancellor Ed Balls complaining during a tumultuous Shadow Cabinet meeting that he was ‘out of the loop’ on the offer – and warned it could prevent a future Labour Government from ever passing a Westminster Budget if Scottish MPs were barred from voting.

The Mail on Sunday poll highlights how much is at stake for the Labour leader. Survation found that across the UK, Labour’s support is running at 35 per cent – a lead of four per cent over the Conservatives – enough to give Mr Milband a Commons majority of 44 if a General Election was held tomorrow.

If Scotland was excluded, Labour would be one point ahead, on 34 per cent, and Mr Miliband’s majority would be slashed to just nine seats.

During his interview, Mr Cameron insisted that the devo max plan in the aftermath of a No vote would be the final ‘resting place’ of devolution, and it would put an end to Scottish National Party leader Alex Salmond’s campaign.

The PM said: ‘The UK is not indifferent, you hear lots of concerns about what it means for them, what it means for our standing, what the consequences would be. More than anything else from the rest of the UK you get arguments of passion rather than arguments about detail, which I hope shows everybody knows how much we care about the UK.

'Scratch an Englishman and you don’t have to go very deep until you find an absolute passionate Briton and someone who believes in the UK.’

This will go down to the wire, it's nerve-racking

The Mail on Sunday poll also found that the referendum campaign has stimulated support for an English Parliament, with 47 per cent backing the measure and just nine per cent opposed.

It also showed strong support for a guarded border between England and Scotland if Mr Salmond signed up to the Schengen agreement, the EU’s open-borders programme.

The Prime Minister will try to push home the No camp’s advantage tomorrow with a major speech in which he is expected to pay tribute to Scotland as ‘a proud, strong, successful nation’ and hail ‘the patriotic vision of a strong Scottish nation allied to the rest of the UK.’

Responding to the eight-point poll lead for the No camp, Mr Salmond said: ‘I think it is wrong because I think there is huge movement – and the movement is towards Yes.

‘We’ve had over the last week panic, heeby-jeebies from Downing Street. The Yes campaign is going to win this campaign, and win it well.’

Balls: Brown’s transfer of tax-raising powers would be a nightmare

Taxing question: Shadow chancellor Ed Balls

Ed Balls has raised fears that handing massive new tax-raising powers to Scotland – as suggested by Gordon Brown – could mean big trouble for a future Labour government.

The Shadow Chancellor privately warned that Scottish Labour MPs might have to be banned from voting on budgets for England and Wales if Edinburgh gained control of income tax North of the border.And that could leave a Labour government at Westminster unable to pass finance legislation for the rest of the UK.

Mr Balls’s concerns came amid predictions that Labour leader Ed Miliband will be ‘fatally wounded’ if Labour supporters in Scotland vote ‘yes’ on Thursday.

A rapid timetable to hand Scotland sweeping new powers over income tax rates and finance in the event of a ‘no’ vote was unveiled by ex-Prime Minister Mr Brown last week in a surprise bid to reduce the likelihood of an independent Scotland.

But at a behind-closed-doors meeting of Labour’s Shadow Cabinet held the following day, Mr Balls warned of grave implications for an incoming Labour government if Mr Brown’s plan went ahead. Mr Balls said: ‘If we give away the whole tax-raising power to the Scottish Parliament, how can Scottish Labour MPs then vote for a Labour Budget in England?’

Currently, 40 of the 59 Scottish MPs are Labour, with pollsters predicting Mr Miliband will need a sizeable Scottish contingent at Westminster to form a government with a workable Commons majority after the General Election next May. A witness at the meeting said: ‘It was clear what Balls was saying.

‘He was wondering out loud how we would get Budgets passed at Westminster without Scottish Labour votes. It’d be a nightmare.’

Mr Miliband had no answer to his Shadow Chancellor’s concerns, according to reports.

Last night, senior Labour figures said there was mounting anger over Mr Miliband’s ‘failure’ to energise the vital Scottish Labour vote to come out against independence.

One said: ‘If Miliband’s been unable to deliver Labour voters in Scotland – and the figures point to that – it’s another nail in the coffin for the party as a whole.