Ed Miliband is fuelling the suspicion that he would rather see the Tories get back into power than work with the SNP by not giving a commitment to the nationalists, according to Nicola Sturgeon.
The first minister of Scotland stressed she has said her party will vote to stop David Cameron's Tories regaining office.
But the SNP leader, speaking in Livingstone, claimed Miliband had not followed suit and risks putting anti-Tory voters off in Scotland.
"I hope to hear him give that commitment before too much more time elapses because as long as he fails to give that commitment he leaves lingering the suspicion that Labour would rather see the Tories get back into power than work with the SNP," Sturgeon said.
The nationalist firebrand also promised SNP MPs would oppose the renewal of the UK's nuclear defence system, Trident, at every opportunity.
Sturgeon made the commitment after taking part in a historic seven-way TV leaders' debate on ITV on 2 April.
An average of three snap polls (ICM, YouGov and ComRes) after the two-hour event showed that most people thought the SNP leader had won, with David Cameron in a close second (22% vs 21%). Sturgeon revealed her email inbox was "overflowing" with messages from well-wishers from across the UK.
The comments come after an inquiry was ordered by Cabinet Secretary Sir Jeremy Heywood into an alleged leaked memo, which apparently showed Sturgeon had told a French ambassador she would prefer Cameron to become prime minister.
The first minister hit back after The Telegraph published the story and claimed the report was "100% untrue".
"I've said that, the French ambassador, who was the other person who was in the conversation, has said that. That really should be an end to the matter," she said.
"In my view the biggest question is who wrote this memo, since the Foreign Office now appear to be denying all knowledge of it. How did it come to contain such an inaccuracy? And how did it get into the hands of the Tory supporting Daily Telegraph? I want an enquiry into this."
The nationalist party is set to take a swathe of seats from Labour at the general election, according to the latest poll from YouGov. The survey, of more than 1,000 respondents, found 46% of people plan to pick Sturgeon's party on 7 May, well above Scottish Labour's 29%.