NICOLA Sturgeon has made clear SNP MPs would vote down a Labour Budget that contained its current austerity proposals but would not seek to turn Ed Miliband out of office, rather force him to rethink his economic plans for Britain.

But this prospect could result in US-style gridlock in which the Labour Prime Minister would potentially have to bring forward fresh proposals repeatedly until he could secure a Commons majority.

During heated exchanges in the third and last Scottish leaders' debate in Edinburgh, Mr Murphy challenged Ms Sturgeon, asking her were there circumstances in which the SNP would vote against a Labour Budget.

She replied: "If Labour puts forward a Budget that imposes more cuts on vulnerable people - as clearly they intend to do - the SNP will vote against it and we will seek to use our clout in the House of Commons to get a fairer deal."

Ms Sturgeon was clear that a vote against a government, under the Fixed Term Parliaments Act, would not bring it down but meant it would have to "go away and rethink" its proposals.

Mr Murphy declared that the "cat was out of the bag" as it was clear the Nationalists would be willing to vote down a Labour Queen's Speech and a Labour Budget. He stressed to achieve a "sense of stability, to get that Labour government, is not to take the risk with the SNP to allow David Cameron back into office".

He insisted: "It's clear after Friday they are willing to bring down a Labour Budget, they're willing to bring down a Labour Queen's Speech; Nicola you did that once before and it let in a Tory government."

But Ms Sturgeon hit back, saying: "In 2009, the SNP minority government had its Budget voted down in the Scottish Parliament by Labour. What did we do? We had to go away and we had to rethink that Budget, we had to talk to Labour and to bring it back a week later and, lo and behold, we got it through. That's what happens in the Scottish Parliament and it leads to better decisions."

During the debate, there were again clashes on the so-called £7.6bn "black hole" should the Scottish Government secure full fiscal autonomy and see the end of the Barnett Formula.

Mr Murphy accused the Nationalists of seeking to load more austerity onto Scots while Ms Sturgeon claimed her opponent was perpetuating a "mythology from a desperate Labour Party", insisting full fiscal autonomy was about getting powers to grow Scotland's economy faster.

Earlier, after the Scottish Labour leader attacked the Tory-led government over welfare sanctions, he was strongly challenged by Ruth Davidson.

The Scottish Conservative leader accused Mr Murphy of an "outright lie" and "making it up", insisting there were more than 100,000 fewer benefit sanctions this year than last.

In a later development, Ms Sturgeon came under pressure over a second independence referendum.

Mr Murphy said he did not want Scotland to "bang on all the time" about a second poll and urged the FM to agree to a five-year moratorium.

To applause, Ms Sturgeon insisted the only people talking about another referendum were her political opponents and that "even if the SNP win every single seat on Thursday it is not a mandate for a referendum on independence, it is a vote to make Scotland's voice heard".

Willie Rennie, the Scottish Liberal Democrat leader, pressed Ms Sturgeon over a second referendum and whether Trident renewal would be a so-called "material change" to spark another poll.

He said: "SNP activists on the doorstep are talking about independence...Their candidates say this election is a stepping stone to independence. So what's clear is that if you vote SNP at this election and every election it's for independence; we should put that divisiveness behind us."

Today, Ms Sturgeon will insist SNP MPs will use their voting strength at Westminster to ensure future UK governments give the NHS the additional investment it needs; a move which would secure the extra £8bn that is required for the NHS in England and see spending for Scotland's NHS increase by a total of £2bn by 2020/21.

Ahead of a campaign speech in Dumfries, she said the Nationalists would reverse Labour's "devotion to the macho cuts agenda of the Tories" and deliver "progressive policies for the benefit of the whole UK".

On the campaign trail, David Cameron continued to intensify the rhetoric against the prospect of a Lab-SNP alliance, saying it would lead to the "road to ruin".

In a BBC interview, he defended his attack on a Lab-SNP tie-up, saying the prospect was "incredibly dangerous for our country and that's why I'm warning so vigorously; if people have got four days to stop it, I've got the answer: if you vote Conservative, it won't happen".

In other developments, Mr Cameron was accused of "ducking" one of the last set-piece events of the election campaign, after it emerged he would not be attending a 2200-strong rally today staged by community organising charity Citizens UK, which will be addressed by Mr Miliband and Mr Clegg.

The DPM said the decision was typical of the "bloodless and uninspiring" campaign run by the Conservatives while Labour said it was an indication that the Prime Minister was unable to defend his record.

But Tory sources said that it had been made clear some time ago that Mr Cameron would be unable to attend due to campaigning commitments elsewhere in the country and that Culture Secretary Sajid Javid would represent the party.