Sir Keir Starmer has said there will be “no deals” with the SNP before or after the next general election.
The Labour leader said there would be “no alliance” with the SNP under any circumstances.
He spoke to journalists along with Scottish Labour leader Anas Sarwar on Tuesday.
“And that’s not the first time I’ve said it. I’ve said it every single time I’ve been asked about it.
“And I’ll tell you why, there is no alliance to be forged with a party that wants to break up the United Kingdom.
“I want a Labour Government and I want to be the Prime Minister for the whole of the United Kingdom.”
He continued: “There is no basis for an alliance with a party whose sole focus is on breaking up the United Kingdom, and therefore it’s not going to happen.
“No deal in, no deal out.”
Nicola Sturgeon has said the next general election will be a “de facto referendum” if her plans for another vote on independence are rejected by the Supreme Court.
Sir Keir said: “It doesn’t change the principal position. We go into that election making a case for change.
“We go into that election making the case for a Labour government for the whole United Kingdom.
“And in the local elections I came to Scotland and the thing that came up everywhere I went was the cost of living.
“People saying ‘I can’t pay my bills, Keir. I’m really struggling to make ends meet, my energy bills have gone up, my wages haven’t gone up, prices go through the roof’.
“Inflation is going through the roof but I want to see a government that can actually do something about it.
“And so we want to fight the next general election on strengthening the economy, turning that around and actually answering what I think is the number one priority issue for most people living in Scotland, living across the whole United Kingdom.”
Mr Sarwar said: “It’s not going to get the fact that no matter what the SNP might say, it is not a de facto referendum.
“It’s not going to be Scotland against England. It’s going to be Boris against Britain.”