Harriet Harman has rejected a proposal to put the next Labour leader up for re-election in three years' time.
The acting Labour leader said she disagreed with the suggestion that the party should have a chance to remove the new leader if they were found wanting before the next general election.
Some MPs, including shadow education secretary Tristram Hunt, had suggested the next leader must seek re-election or re-endorsement before 2020.
But Ms Harman told the Observer newspaper that once a leader was elected under the new "one-person, one-vote" system, it was "for them to be getting on and doing that job" for five years.
Former deputy Labour leader Margaret Beckett will now lead a commission to examine "in a forensic way" the reasons behind Labour's election defeat, she added.
Ms Harman told the newspaper: "We want at the end of this truth and reconciliation commission to have a better and honest understanding of why we ended up in this situation, but we need to be united and coherent in order to be attacking the Government and also to make sure we are in a united position to go forward."
Andy Burnham, Yvette Cooper, Liz Kendall and Mary Creagh have announced they will stand for the Labour leadership.
They must now get the support of 35 of the party's MPs to take part in the contest, which will be decided in September.
Meanwhile, Ms Harman said she believed voters in the general election had only decided late on in the campaign to reject the party.
There were "a large number" of undecided voters who had made up their minds at the last minute and "stuck with the devil they knew", she told the Observer.
"There is some anecdotal information about people hovering outside the polling stations thinking 'Should I do this or that?' It is down to us to find out why we couldn't convince people to trust us," she said.