Politicians who begged Scotland to stay in the Union should not complain if the SNP calls the shots at Westminster after the election, Nicola Sturgeon said last night.

Polling last night showed that the nationalists are extending their lead – threatening to all but wipe out Labour north of the border.

But Ed Miliband yesterday refused four times to rule out going into a power-sharing agreement with the SNP if its MPs hold the balance of power for the first time.

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Boy, are we doing well: SNP leader Nicola Sturgeon campaigning in Irvine, Ayshire yesterday

During a visit to Lincoln, the Labour leader was asked repeatedly whether he was prepared to dismiss the idea of any pact.

While he has rejected a formal coalition he did not address questions as to whether he was prepared to go into Downing Street propped up by Miss Sturgeon on a ‘vote-by-vote basis’.

The SNP leader used a BBC TV debate on Thursday night to offer to ‘lock David Cameron out of Downing Street’ – even if Labour has fewer seats than the Tories in a hung parliament.

She dominated the so-called ‘challengers’ debate’, and was deemed the best performer by 35 per cent of viewers to the Labour leader’s 29 per cent.

Yesterday she said: ‘During the referendum campaign last year, we were told repeatedly by politicians who were trying to persuade Scotland to vote no – and they succeeded – that Scotland was an integral part of the UK, that our view mattered,’ Miss Sturgeon said.

Sturgeon dominated the so-called ‘challengers’ debate’ on Thursday and her party is threatening to all but wipe out Labour north of the border

‘It’s completely unacceptable now for those politicians to say, when Scotland is perhaps going to make its view heard by voting SNP, to say your voice can’t be heard in the UK.’

She said her MPs – who could number as many as 50 after May 7 – would vote to roll back reform of the NHS in England.

Traditionally, Scottish nationalists have not voted on health and matters that do not affect their constituents, since they are controlled in Scotland by the Holyrood parliament.

‘Ed Miliband knows as well as I know that if there is an anti-Tory majority after the election, then the only way David Cameron gets back into Downing Street is if Ed Miliband is prepared to stand back and let him back in Downing Street,’ Miss Sturgeon told Sky News.

‘If Ed does that, then Labour voters in Scotland will never ever forgive him and I suspect Labour voters across the UK will never forgive him. Is he really saying that he’d rather watch David Cameron back into Number 10 than work with the SNP to keep him out of Number 10? That’s for him to answer.

‘I’m fighting for more progressive politics across the UK and I’m fighting to get rid of the Tories but crucially to see the Tories replaced with something better, something bolder something that’s going to deliver progressive policies for ordinary people not just in Scotland but all of the UK.

‘Our MPs will vote for a bill to restore England’s NHS to its founding principles, ensuring it remains the accountable public service it was always meant to be – and protecting Scotland’s health budget in the process.’

SNP activists insist Scotland has long had to put up with Tory governments, even though the party currently has only one MP north of the border.

David Cameron warned last night that voters across the UK would face higher taxes to fund profligate public spending if Labour does a deal with the SNP.

The Prime Minister said Mr Miliband must ‘level with the British people’ and admit that his ‘only route into Number 10’ is with the support of the SNP.

Speaking on a campaign visit to the West Midlands, Mr Cameron said: ‘The fact is that Labour cannot win a majority on their own. They can only get into Downing Street with the support of the SNP.

‘What is it that the SNP want? They want more borrowing, they want more taxes, they want more unlimited borrowing – they want all the things that got us into this mess in the first place.’

Miss Sturgeon’s predecessor as Scottish first minister, Alex Salmond, who is bidding to return to the Commons, suggested that an SNP deal with a minority Labour government was ‘probable’.

He told ITV News a vote-by-vote arrangement with Mr Miliband would be much more likely than any other agreement.

‘What I’ve said and what Nicola has said, more importantly, confidence and supply is possible, but vote by vote is probable.