The leader of the SNP has told the BBC that her MPs will vote on English health matters if it helps to protect the Scottish NHS.
Nicola Sturgeon's comments come the day before a bill on more tax and spend powers for Scotland is published.
They also add to the heated debate about English votes for English laws.
The Tories want English MPs to have sole say on English laws. Labour, which has 40 Scots MPs, rejects that view, while the Lib Dems back limited change.
The SNP has six MPs in the House of Commons and in practice they do not vote on non-Scottish legislation.
However, in an interview with BBC political editor Nick Robinson, Ms Sturgeon warned that if there were moves to "further privatise" the NHS in England, SNP MPs would vote to stop them.
She explained: "On health, for example, we are signalling that we would be prepared to vote on matters of English health because that has a direct impact potential on Scotland's budget.
"So, if there was a vote in the House of Commons to repeal the privatisation of the health service that has been seen in England, we would vote for that because that would help to protect Scotland's budget."
Ms Sturgeon said it was a "self-interested" position to take because if public funding of the NHS in England was reduced then that would have a "direct knock-on effect to Scotland's budget and our ability to protect the funding of Scotland's health service".
Scottish Labour Party leader Jim Murphy dismissed Ms Sturgeon's comments saying that the SNP were spending less on the NHS "than even David Cameron".
He added: "There is a way of getting a better health service, a way of getting more money for Scotland's health service, and that is by voting for the Labour Party."
In September last year, the electorate in Scotland voted against independence.
Immediately after the poll Prime Minister David Cameron promised more powers to Scotland and appointed Lord Smith to work out the detail.
The Smith Commission made a series of recommendation at the end of November and on Thursday the draft Scotland Bill will be published.
Ms Sturgeon, who became leader of her party in December, said she would be "watching closely" to make sure the commission's plan was "delivered in full".
Despite being on the losing side of the independence referendum, the SNP has seen its membership mushroom and polls are predicting big gains at the General Election on 7 May.
Political pundits are now saying that if there is another hung parliament at Westminster and the nationalists increase their seats, they could be power brokers.
Ms Sturgeon has ruled out doing any deal with the Tories but her party could enter into talks with Labour.
She told Nick Robinson that she was "instinctively not too enthusiastic" about a formal coalition but "tended" towards a vote-by-vote deal.
'Not in competition'
However, MP Mr Murphy said that if voters in Scotland chose the SNP at the ballot box "in any number" they would "reduce the size of the Labour Party and increase the chances of David Cameron holding on to power".
He added: "You'll end up with Labour and the SNP being on the same side, but the same side on the opposition benches of the House of Commons. If you want a Labour government, then vote Labour."
Ms Sturgeon believed that an SNP, working with Plaid Cymru and the Greens, would be a "moderate and progressive force" at Westminster.
She also said she was looking forward to seeing former party leader Alex Salmond winning a seat in the House of Commons.
Ms Sturgeon added: "I and Alex Salmond are not in competition - we are on the same side, we are on the same team working together."
By Nick Robinson, BBC political editor
I have been speaking to Scotland's First Minister Nicola Sturgeon about how SNP MPs will vote after the next election.
With the polls predicting a doubling in the party's support and, potentially, a huge increase in the number of their MPs this really matters.
Specifically, Nicola Sturgeon says that SNP MPs will vote on the English NHS. Given that the Scottish government has complete control of the Scottish NHS, this is likely to fuel calls for "English votes for English laws".
She says the move is necessary to protect the Scottish NHS from the knock on effects of cuts and privatisation in England.
Traditionally, the SNP position at Westminster is not to vote on matters which don't impact on Scotland unless they have a direct impact on the budget of Scotland via the so-called Barnett funding formula.
In recent years they voted to oppose the creation of Foundation Hospitals and the introduction of student tuition fees.
By Brian Taylor, BBC Scotland political editor
Nicola Sturgeon speculates upon the events which might provoke the SNP to break their self-denying ordinance and vote in the Commons on matters affecting England.
One such, she suggests, would be to "restore" the NHS in England in order to protect the Barnett-determined Scottish budget.
This, she says, would involve countering the trend towards privatisation which she detects south of the border with, she forecasts, an eventual reduction in public provision.
Two caveats first.
One, this argument replicates the case which the SNP made during the referendum - to the exasperation of opponents who said that the NHS was devolved and, consequently, not a salient issue during a discussion of the Union.
Two, the self-denying ordinance is not universal.
For example, in the past, SNP MPs have taken part in votes on the issues of foundation hospitals and university tuition fees even although there was no direct impact upon Scotland.
So, strictly, the offer with regard to the NHS is not entirely novel.