Labour supporters in England would be "appalled" if their party allows the Tories back into government and rejects a "progressive alliance" with the SNP, Nicola Sturgeon has said.
With polls narrowing in the run-up to June 8, Scotland's First Minister said it was "no longer inevitable" Theresa May will be returned as prime minister with an increased majority.
Labour leader Jeremy Corbyn has rejected a formal deal with the SNP to lock the Tories out of power at Westminster in the event of a hung parliament - but there is still a prospect the parties could work together informally on a vote-by-vote basis.
Mr Corbyn has insisted: "'We are not doing deals, we're not doing coalitions, we're not doing any agreements."
Meanwhile, Scottish Labour leader Kezia Dugdale said there would be "no need to negotiate" with the SNP because the party would face the prospect of backing a minority Labour administration or "letting the Tories back in".
Ms Sturgeon told BBC Radio 4: "If there was to be a hung parliament, if the parliamentary arithmetic allowed it, then I would want the SNP to be part of a progressive alternative to a Conservative government.
''Not in a coalition, I don't envisage any formal coalitions, but on an issue-by-issue basis to put forward progressive policies and to see a progressive agenda."
With the Tory lead in the polls shrinking, and with less than a week to go until the election, Ms Sturgeon said Scotland was once again "taking centre stage" in UK politics.
She added: "I think many people south of the border would be appalled at how Scottish Labour is behaving where they seem to be more determined to somehow stop the SNP than they are to stop the Tories, which I think would be inexplicable to most people in the Labour Party south of the border."
A surge in Conservative support north of the border has suggested there could be between six and a dozen Scottish Tory MPs after June 8 - in what would be the party's best Westminster election result for two decades.
Ms Sturgeon said voters have a "real opportunity in Scotland to hold the Tories in check and make sure we don't send Tory MPs to boost the majority and give Theresa May a rubber-stamp".
With some reports suggesting pro-Union parties in Scotland are working together to defeat the SNP by limiting campaigning in some areas, Ms Sturgeon said: "I think it is shameful that Labour appear to be in cahoots with the Tories in Scotland.
"Instead of working to hold a Tory government in check, you have Scottish Labour appearing to be in talks with the Tories or appearing to be in informal arrangements with the Tories to boost a Tory majority.
"I think most people would be appalled at that.
"As this campaign has gone on, I think more and more people have seen what the consequences of a Tory government with an increased majority would be and they don't like that."
Emily Thornberry, the shadow foreign secretary, told The Times newspaper that Labour would opt to form a minority government rather than do deals with other parties.
"If we end up in a position where we are in a minority, we will go ahead and put forward a Queen's Speech and a budget," Ms Thornberry said.
"If people want to vote for it, then good. If they don't want to vote for it, they are going to have to go back and speak to their constituents and explain to them why it is that we have a Tory government instead."
Speaking on the campaign trail in Edinburgh, Ms Dugdale agreed, stating: "The reality is that in a minority Labour administration situation there would be a Labour budget underpinned by the commitments we have in our manifesto and it would be there and presented for the SNP to either vote for or vote down.
"In that situation, if they vote for it, great that's what will be delivered, if they vote against it we would find ourselves in a position where we would have a Tory government all over again.
"So, there is no need to negotiate with the SNP.
"It is down to them to either live by their anti-austerity credentials that they are so keen to talk about or let the Tories back in."
Asked if a second independence referendum could become a bargaining chip in such a scenario, Ms Dugdale said: "Categorically not, we've made it abundantly clear in our manifesto that we think a second independence referendum is unnecessary and unwanted."
The Scottish leader also endorsed Mr Corbyn, adding she was not surprised by his progress in the polls.
"He is that authentic voice, people like that he is not a scripted politician, that he says what he thinks," she said.
"Given 20 minutes to freestyle, he can give you an impassioned take on Labour values and the difference that a Labour government can make.
"That's why when he does a 10-second TV clip sometimes he gets himself into a little bit of difficulty, in your eyes not mine, but the reality is people like what they see."