Labour erroneously attempted to win the general election without winning over any Tory voters, according to Jim Murphy.
The former Scottish Labour leader blasted the party's "wishful thinking", which saw them lose 24 seats and the Tories secure a shock majority in the House of Commons.
Murphy, making the final speech of his political career at the think-tank Policy Exchange, said: "There was a view over the past five years – mistaken as some feared and has now been proven – that there was a route to victory without winning over any Tory voters.
"Some thought that a coalition of non-Tory (as opposed to anti-Tory because those are two different things) voters could be brigaded behind Labour, while the Conservatives were struggling to see off Ukip on the right. What was a theory is now exposed as wishful thinking".
But the former East Renfrewshire MP, who lost his seat to the SNP at the election, stressed that he did not think the defeat was "all Ed Miliband's fault" and criticised the former Labour leader's detractors.
"Of course he and I had our private differences – which sometimes unfortunately spilled out – but I served in the shadow cabinet with him until I became Scottish leader. And then I had his full support for the campaign I chose to run," Murphy said.
"In truth Ed was true to his convictions and took the party in the political direction that he pledged he would when he stood for the leadership, supported incidentally by many of those who now unfairly chastise him."
Murphy, who oversaw Scottish Labour's annihilation at the election when the party saw its number of MPs plummet from 41 to just one, warned that the party have a "mountain to climb" north of the border.
"To win a country you need a compelling vision of its future. Compelling, inclusive – and for a party of the centre one based on fairness. We sometimes lacked that across the UK."
The speech comes after all four Labour leadership hopefuls (Andy Burnham, Yvette Cooper, Jeremy Corbyn and Liz Kendall) received enough nominations (+35) from fellow MPs to get on the party's ballot paper.
The first hustings in the next stage of the race will be hosted by BBC Two's Newsnight on Wednesday (17 June) in Nuneaton, a swing seat Labour failed to win at the election. Meanwhile, nominations for Labour's deputy leadership contest close at 12 noon on the same day.