Murphy: Sturgeon's full fiscal autonomy revelation was first genuine big moment of election campaign in Scotland

Under full fiscal autonomy, Scotland would be responsible for raising enough cash to cover all of the country's public spending - with Labour claiming research by the Institute for Fiscal Studies shows this would leave a £7.6 billion blackhole in the nation's finances.

Mr Murphy challenged the SNP leader on the issue in two back-to-back TV leaders' debates, which he said had left Labour with a "spring in our step".

The party north of the border is trailing well behind the nationalists in opinion polls, which suggest it could lose the majority of its 40 Scottish seats in the General Election on May 7.

Ms Sturgeon's comments on full fiscal autonomy were "a very big moment" in the campaign, the Scottish Labour leader said.

"In Scotland it's the first genuine big moment of the campaign and we're going to press on with this argument."

Ms Sturgeon said in a BBC Scotland debate that she would like full fiscal autonomy for Scotland "as quickly as the other parties agree to give it".

Mr Murphy asked the SNP leader: "Would your MPs vote for it next year?''

She replied: ''I would vote for it, would you support it?''

Mr Murphy said Labour would ''absolutely not" back the change, adding his party would fight for the Barnett formula, which determines how public cash is distributed across the UK, to remain in place.

The Scottish Labour leader, who was campaigning in Cumbernauld, North Lanarkshire, said: "What we saw last night in the debate was Scotland's First Minister admitting under her plans Scotland would be £7.6 billion worse off - it's full fiscal austerity.

"We will oppose Tory austerity by the front door and we will also oppose the SNP's austerity by the back door. We will keep the Barnett formula today, tomorrow, forever."

Mr Murphy continued: "There's no way on earth we will vote for full fiscal austerity, or full fiscal autonomy.

"It's more than £7 billion of cuts for Scotland. Why would anybody who loves our country want to cut that £7 billion?"

He also argued that changing the way services in Scotland are funded would mean the end of the UK pensions and benefits system north of the border.

"If the SNP have their way, you would have the abolition of the UK state pension here in Scotland and the abolition of UK benefits," he claimed.

"Full fiscal autonomy was always a nice-sounding advertising slogan in search of a policy - well, it's found a policy now and it's £7.6 billion of cuts and the end of the UK pension system.

"Between now and polling day, no one is going to be left in any doubt that if you vote for the SNP, you are voting to opt out of the UK state pension system. That is awful news for Scottish pensioners."

He insisted only Labour could prevent Conservative Prime Minister David Cameron winning a second term in office but added: "In this race, David Cameron can't win seats in Scotland, he has to have someone to win seats for him.

"In this contest he's hoping the SNP can defeat Labour in Scotland and that it propels David Cameron back into Downing Street.

"I'm confident after these debates that we have a spring in our step, we have the arguments on our side, we have the truth on our side, and we're the party with momentum and self-belief."

Liberal Democrat Business Minister Jo Swinson also hit out at SNP plans to change the way services in Scotland are funded, claiming this could cost Scotland some £40 billion over the five year parliamentary term.

She said: "You cannot have strong public services without a strong economy. Scottish Liberal Democrat plans to deliver £8 billion in NHS funding in England and an extra £800 million for Scotland is only possible because we have taken responsible decisions in government and got the economy back on track.

"Yesterday we heard Nicola Sturgeon pledge that SNP MPs would vote for full fiscal autonomy in the first year of a new parliament. She needs to explain why she backs a plan that would leave a £40 billion black hole in Scottish finances. How on earth would that help us fund our NHS properly?"

Ms Swinson, who was campaigning in the Ardardan Estate in Cardross: "The other parties cannot match our commitment to fund the NHS in Scotland because they cannot pay for it.

"Labour and the SNP would risk the economy and jobs with their massive borrowing and the Tories will cut, cut and cut again.

"Whether they are borrowing too much or cutting too much, the other parties' economic plans pose a grave threat to the health of our NHS and other key services. We know that the SNP have already fallen behind England and the rest of the UK on investment in our NHS and services are under massive strain. Scotland deserves better than that.

"You don't have to choose between a strong economy or strong public services. You can have both, but only with the Liberal Democrats."

SNP deputy leader Stewart Hosie hit back at Mr Murphy and said: "Labour are in the same place as the Tories, which is why so many people are switching to the SNP. They are proposing more cuts, and defending Westminster controlling over 70% of Scotland's revenues and 85% of the welfare spending in Scotland - people in Scotland want the vow of extensive new powers delivered."

He added: "The whole purpose of financial autonomy is having control of our employment, taxes and spending to grow our economy instead of facing Tory and Labour cuts.

"Labour are just trying to hide their plans for £30 billion of real cuts.

"Scotland's economy is strong - by 2020 our onshore revenues will grow by £15 billion - and with more powers we could achieve more

"The UK deficit was £98 billion last year and over the five years to 2013-14, the UK's cumulative deficit has been worth over £600 billion. In two of the last four years, Scotland's deficit has been less than the UK's, and in each of the last 34 years Scotland has paid more tax per person than the rest of the UK.

"Labour's referendum alliance with the Tories is clearly still in operation."