Labour leader Jim Murphy has urged Nicola Sturgeon to “stop the spin and come clean on her plans for a second referendum”.
Former prime minister Gordon Brown warned that the SNP was seeking a strong representation in Westminster in order to “twist the arms” of MPs into granting another referendum.
But Ms Sturgeon insists this election is not about another referendum and says she will not be pressured into it by her rank-and-file if she does not think Scotland is ready for a re-run.
Speaking ahead of a visit to a school basketball court, Mr Murphy said: “A vote for the SNP will put Scotland on the road to a second referendum.
“Nicola Sturgeon promised the referendum was a once in a lifetime event, then it was once in a generation, then it was 15 years and now it’s being planned for the 2016 manifesto.
“The priority for SNP MPs after this election won’t be improving the economy, it will be planning the second referendum campaign.
“A vote for Scottish Labour will put Britain on the road to a fairer economy. With Scottish votes Labour can get the Tories out of power, stop austerity, ban zero-hours contracts and end the need for food banks.
“Only Labour can stop the Tories being the largest party. With the Tories and Labour neck and neck in England, in Scotland we should vote Labour to make sure the Tories are not the largest party.
“The choice is simple – the road to a second referendum with the SNP or the road to a fairer economy with Labour.”