Sir Keir Starmer said he was “tattooing on my forehead” that Labour would not do a deal with the SNP after the next general election.
The Labour leader rejected any form of pact or coalition with Nicola Sturgeon’s party in the event of a hung parliament if his own party failed to win a majority.
Sir Keir also challenged the SNP on whether it would “prop up a Tory government” in the event Labour was struggling to form a government after the vote expected in 2024.
“I know what’s going to happen as we get towards the election. There will be lots of graphs saying this number of seats, that number of seats,” he told LBC on Monday.
“That’s why I’m saying up front – tattooing it my forehead – no to deal with the SNP,” said Sir Keir.
The Labour leader added: “We are not borrowing seats from the SNP and getting over the line. No deal … We are not going to do it.”
The Labour leader has been repeatedly grilled about the prospect of doing a deal with the SNP or the Liberal Democrats, as the Conservatives play up the prospect of “stitch-up”.
His strongest rejection of any collaboration with the Scottish national party, Starmer added: “The SNP will have to make their minds up.
“If they want to bring down an incoming Labour government, and prop up a Tory government, then that’s their choice, and that’s what they will have to sell back in Scotland. Good luck with that one.”
Ahead of the 2015 election, David Cameron’s Tories stoked fears that Labour’s Ed Miliband could be propped up by Ms Sturgeon’s SNP as part of a “coalition of chaos”. The tactic was used again ahead of elections fought under Jeremy Corbyn’s leadership.
The latest comments about hung parliaments come as the centre-left pressure group Compass launch a tactical voting campaign aimed at ousting Tory MPs in 62 seats.
The Win As One campaign is encouraging voters to think strategically in dozens of seats described as “progressive tragedy” because the combined progressive vote outnumbers right-wing parties despite Tory wins.
Meanwhile, Sir Keir accused the government of showing a “profound lack of leadership” over the strikes – but said the pay demand by nurses was “probably more than can be afforded”.
Labour’s shadow health secretary Wes Streeting branded health secretary Steve Barclay’s approach to the NHS pay dispute a “complete joke”.
The Labour frontbencher accused ministers of “spoiling for a fight” and a “betrayal of patients” by refusing to properly engage with unions.
Mr Streeting doubled down on his criticism of the British Medical Association (BMA) health union, saying it has made “unreasonable” complaints about Labour’s NHS plans.
