Kezia Dugdale has said she would reject any deal, coalition or pact with the SNP to get Labour into power at Westminster.
The Scottish Labour leader said a deal with nationalists “just wouldn’t work”.
Shadow Scottish secretary Dave Anderson has said Labour should consider a coalition with the SNP to prevent another Conservative government at the next general election.
Writing in the Daily Record, Ms Dugdale said: “There has been some talk from people based elsewhere in the UK that in the future Labour and the SNP could do some sort of deal to get into power at Westminster.
“On a simplistic level that might sound appealing, but you only have to scratch the surface to see why that just wouldn’t work.
“Labour is a socialist party. The SNP most certainly aren’t. Sure there are some socialists in the SNP, but that is always overtaken by their nationalism.”
She said the SNP campaigned for independence when Labour created the NHS, introduced the minimum wage and throughout the last Labour government which “lifted millions of children and pensioners out of poverty”.
She added: “If we were ever privileged enough to be in a position to put a programme for government in front of MPs in the future, SNP MPs would have to choose to support a Labour or a Tory government.
“I’d hope they would use the opportunity to back Labour’s progressive policies and block the Tories. But a deal, coalition or pact? No thanks.”
On Sunday, Mr Anderson’s predecessor Ian Murray, Labour’s only MP in Scotland, accused Jeremy Corbyn and his allies of a lack of understanding of the political dynamic in Scotland by failing to rule out an alliance with the SNP.