David Miliband has claimed the Labour Party has not been further from power since the 1930s as part of a stinging attack on Jeremy Corbyn.
The one-time favourite for the Labour leadership branded the party "unelectable" and said Mr Corbyn's "half-hearted" campaigning for Britain to remain in the European Union was "a betrayal of millions of working people".
Meanwhile, Mr Corbyn has insisted his leadership style is unlikely to change should he beat challenger Owen Smith and be named the winner of the contest for the Labour top job on Saturday.
"Sadly for everyone, it's the same Jeremy Corbyn," he said.
Voting in the leadership battle closed at noon on Wednesday and Mr Corbyn is the overwhelming favourite to take victory.
But Mr Miliband, the former foreign secretary, has set out a damning assessment of the incumbent.
Writing in The New Statesman, he said: "The main charge against Jeremy Corbyn is not just that his strategy is undesirable because it makes the party unelectable. That is only half the story.
"The real issue is that his strategy makes the party unelectable because it is in many aspects undesirable."
Mr Miliband is particularly critical of Mr Corbyn's "egregious" stance on foreign policy.
"But the electorate can see through the domestic policy, too," he said.
"Nationalisation cannot be the answer to everything; anti-austerity speeches cannot explain everything; corporate taxation cannot pay for everything.
"It doesn't add up. It wouldn't work. People are not stupid."
Mr Miliband said it is "disastrous" that people are branded "closet Tories" or "Tory Lite" if they disagree with Mr Corbyn.
The president of the International Rescue Committee claimed Labour had slipped from a party fit for government to a "secondary influence on national decision making", adding: "We have not been further from power since the 1930s."
Meanwhile, Mr Corbyn has promised to work with Mr Smith and other internal critics if he is confirmed as leader on Saturday in order to heal the rift which saw him lose a confidence vote among the parliamentary party by a margin of 172-40.
Mr Smith has said he will not accept a place in Mr Corbyn's shadow cabinet if the current leader retains power.
In a statement following the close of polls, Mr Corbyn said: "As far as I am concerned, the slate will be wiped clean this weekend.
"If I am re-elected leader, I will reach out to and work with all Labour MPs to form a broad and effective opposition to this divisive and floundering Tory government."
He added: "All Labour party members and MPs have a responsibility to work within the democracy of our party and respect the leadership of whoever is elected."
But Mr Corbyn has set himself against proposals from deputy leader Tom Watson to return to the electoral college system which gave MPs and unions more say over the choice of a leader.
"I think we have to stick with one-member one-vote," he said.
Labour's ruling National Executive Committee has put off a decision on MPs' demand to be allowed to elect members of Mr Corbyn's shadow cabinet.