The leader of the SNP said the party would only press for another independence referendum if something "material changed".
Nicola Sturgeon made the comment in a TV debate of Scottish party leaders.
The first minister's comments came as she was asked if plans for a fresh vote on Scotland's future would be in her 2016 Holyrood election manifesto.
Scottish Conservative leader Ruth Davidson said the SNP had ruled out another referendum "for a generation".
Jim Murphy, Scottish Labour leader, emphasised that May's general election was not about independence.
The issue of a future referendum was raised during a BBC Scotland TV debate in Aberdeen, which was broadcast on Wednesday evening on BBC1 Scotland and the BBC News channel.
David Cameron had pledged an in/out referendum on Europe by the end of 2017, if the Conservatives win the election.
Ms Sturgeon had previously said the Westminster election was not about independence, but during an STV debate on Tuesday added that using the 2016 Holyrood elections to bring about a second independence vote was "another matter".
She told the BBC Scotland debate she was "not planning or proposing another referendum", but when asked about the 2016 referendum, responded: "Something material would have to change in terms of the circumstances or public opinion before I think it would be appropriate to have a proposal for a referendum."
By Brian Taylor, BBC Scotland political editor
"We learned that a further early referendum on Scottish independence is notably less likely. How so?
"Nicola Sturgeon does not want to talk referendum at this UK General Election. It tells her that it polarises opinion in Scotland, it divides the nation - she wants to stick to the offer that she makes of providing a strong voice for Scotland at Westminster in total - appealing to whole of the Scottish people.
"She says the referendum on independence is a matter to be determined by the Scottish parliamentary election next year.
"Now she said tonight that there would have top be some material change, not just the passage of time before she would countenance putting out a promise in the manifesto for that early referendum.
"She gave the example of Britain quitting the European Union against Scottish opinion - but that is scheduled for 2017. She has to take a decision on that manifesto in 2016.
"My understanding is that at this stage it is less likely than otherwise that there will be that pledge for a further referendum in that manifesto."
Ms Sturgeon added: "Perhaps if the Tories wanted to drag us out of the European Union against our will, for example. That might change the circumstances."
She also said it would be "completely outrageous" to "rule out forever and a day the Scottish people getting the chance to choose".
The first minister went on to say there was a "triple lock" on a further independence referendum, adding: "Before it's inserted in a manifesto, something has to change. Then people have to vote for the manifesto - if it is in it - and then people have to vote for independence."
Ms Davidson said she did not see a situation where, if another referendum was to be held, her party would block it.
But she added: "We were promised - and we would feel the betrayal very deeply - time after time after time, by Nicola, by Alex Salmond, by John Swinney, by all of her MSPs, by all her MPs, all her MEPs and all her councillors that this was - and I quote the SNP poster 'their one opportunity'.
"When were told it was once in a generation - we were told by the end of the campaign it was once in a lifetime, if she coming back next year in her manifesto with this and that was the question she didn't answer last night and I would like an answer to it this evening."
Mr Murphy told Ms Sturgeon: "You've gone from being the proud co-leader of the big 'Yes' campaign to being the head of the 'mibbes aye mibbes naw' campaign.
"I'm looking forward to seeing that badge or slogan or placard."
Scottish Liberal Democrat leader Willie Rennie said: "Just imagine if we had a different vote last September. I think there would be blind panic as a result of what has happened in the North Sea.
"To have our economy, not wholly, but largely dependent on the volatile resource of the North Sea I think would have caused absolute chaos to our public services, to our pensions, to our teachers, to our hospitals.
"Nicola Sturgeon has got a nerve to continue to argue for a policy that was soundly trounced in the referendum."
Scottish UKIP MEP David Coburn said: "If we had listened to Ms Sturgeon and her crew, quite frankly, we would be bankrupt, we would have nothing, the country would be finished."
Scottish Green co-convener Patrick Harvie said: "The phrase a moment ago was 'control over our own economy', but let's remember that so much control over our real economy - infrastructure, oil, energy - has been handed over to a tiny number of vast multinationals.
"That's the kind of control that we need to get back so that our economy itself is democratically accountable."
During the hour-long programme, Ms Sturgeon also confirmed she would introduce full fiscal autonomy for Scotland within a year if given a chance, despite opposition warnings that it would leave an immediate £7.6bn hole in Scotland's economy.
Asked by Ms Sturgeon whether he would vote for it, Mr Murphy said: "Absolutely not, and let me tell you why.
"This is the idea that we cut ourselves off from sources of taxation across the UK.
"After the difficult time that Aberdeen and the north east of Scotland have been through, the idea that we voluntarily give up the pooling and sharing of resources, the ability to transfer money across these islands - I don't think it makes sense."
Ms Davidson said: "Full fiscal autonomy, right now if we vote for it next year, would mean that we had billions of pounds less in Scotland to spend on welfare."