Sturgeon: getting more SNP MPs is interim step to next indyref

But she will also seek alliances with parties such as Plaid Cymru and the Green Party to be a "progressive force in the House of Commons" and vote on issues that the SNP regard as in the best interests of Scotland, she told the BBC's Andrew Marr Show.

Her desire to send a large number of SNP MPs to Westminster to be a strong voice for Scotland is an "interim" measure until the next independence referendum, she said.

Ms Sturgeon also repeated her call for the proposed in/out referendum on Europe to be subjected to a "double majority", where each constituent nation must vote Yes before a withdrawal is approved to prevent any part of the UK being taken out of the EU "against its will".

She said: "I think there is a very, very strong case for English votes for English laws...but if you take health for example we know that because of the Barnett Formula that decisions on the English health service impact on the budget of the English health service and that has a direct knock on effect for Scotland's budget.

"That is why it would be perfectly legitimate, in those circumstances, for SNP MPs to vote on matters like that.

"Now we will take these decisions based on what we think is in the best interests of Scotland.

"But I would also want to see SNP MPs being a constructive and progressive force in the House of Commons looking to make alliances for progressive politics where we can, working with parties like Plaid Cymru and the Green Party to be a progressive force.

"I think the Westminster establishment really needs some progressive forces at its heart."

She added: "I want Scotland to become independent, you have probably noticed that over the past wee while, but that will only happen when a majority of Scotland want that to happen.

"In the interim, what is really important is Scotland's voice is heard loudly at Westminster."

Ms Sturgeon dismissed suggestions that Scotland could declare independence without a referendum.

"The only way for Scotland to become independent is for a majority of people in Scotland to vote for independence in a referendum - that is the route to independence," she said.

"Do I think that will happen? Yes I do.

"I can't sit here right now and tell you exactly when that will happen. It will happen when there is a demand in Scotland for a referendum.

"One of the things that I think will seriously concern people in Scotland is if we do have a European in/out referendum and we may have a scenario of the UK voting to come out and Scotland voting to stay in."

She said there should be a "double majority" in the EU referendum.

"During the referendum on Scottish independence we were told repeatedly that the UK was a 'family of nations' and we were an 'equal partnership'," she said.

"Well, if that is true then I think on an issue like whether or not we are members of the EU, in order for the UK to come out there should require not just a vote for that across the UK but a vote for that in each of the component parts of the UK.

"I will be pressing that case in order to make sure that no part of the UK could be taken out of the EU against its will."

Ms Sturgeon confirmed the SNP will vote on English tax issues and said a Labour government dependent on SNP support would lead to "more progressive decisions for the whole of the UK".

"If you look at polls in Scotland, as well as showing a substantial SNP lead they show that the most popular outcome in Scotland in the general election is a Labour government dependent on SNP support," she said.

"Because I think people recognise that would lead to better decisions that were not just, perhaps, in the interests of Scotland but more progressive decisions for the whole of the UK."

She added: "As long as we are funded in Scotland as we are at the moment of course SNP MPs will vote on tax issues, because those decisions affect the budget of the Scottish Parliament.

"Full fiscal control for the Scottish Parliament is not yet being proposed."

Scottish Labour leader Jim Murphy has not ruled out working with the SNP after the general election.

But the East Renfrewshire MP said "it would be wrong" to back the abolition of the Trident nuclear deterrent in exchange for SNP votes.

His deputy Kezia Dugdale MSP she said she would "no qualms whatsoever" about going into partnership with the SNP after the May election.

Speaking on the BBC's Politics Scotland, Mr Murphy said: "I think it's important that parties work together, it happens all across Scotland in local authorities, but what Kez is talking about is the same thing that I've been talking about which is that we're not expecting, we're not planning, we don't want, and we don't need a coalition with the SNP.

"The fact is that come the UK election here in Scotland in May it's really a choice of whether David Cameron hangs on to power or whether we can get David Cameron out of Downing Street, and by voting SNP, unfortunately, a lot of Scots if they do so, run the risk that by accident they ensure that David Cameron holds onto power."

Ms Sturgeon said she hopes to convince Labour to take "a different view and a different decision on Trident" if the SNP gets the chance to work in partnership at Westminster.

But Mr Murphy said: "You make a decision on a nuclear deterrent not about playing footsie about possible coalitions with other parties and all that sort of stuff.

"The things about the nuclear deterrent aren't about negotiations with other parties on hypothetical coalitions in the House of Commons, it's about negotiation with other nuclear states to ensure that the world is nuclear free.

"The nuclear deterrent is too important to get involved in that sort of horse trading on the nation's safety. I want a world free of nuclear weapons, you should negotiate that away with other nuclear powers, not negotiate it away for party partisan games."

Scottish Labour finance spokeswoman Jackie Baillie has warned that Ms Sturgeon's ambition for "full fiscal autonomy" would mean ditching the Barnett formula which calculates Scotland's share of UK spending, and leave Scotland exposed to the economic impact of volatile fluctuations in oil prices.

"The First Minister today confirmed that the SNP want this general election to be the end of the Barnett formula, which would rip billions out of Scottish public services," she said.

"Their plan is dangerous for Scotland. Barnett allows us to benefit from the shared and pooled resources and revenues of the whole UK."