Jeremy Corbyn has driven more of a wedge between rival Labour factions by threatening re-selection, the party's former acting leader has said.
Members and MPs are being set against each other in a further move that underlines why the leader must be ousted, Harriet Harman warned.
During his leadership campaign launch, Mr Corbyn confirmed that all Labour MPs would face re-selection when new parliamentary boundaries - reducing the number of seats from 650 to 600 - come into force in 2018.
The move would pave the way for his supporters in the party's grassroots to push out critics by replacing them as Labour candidates.
Ms Harman told the BBC Radio 4 Today programme: "It's not clear whether what Jeremy Corbyn was saying was that there's going to be new measures in relation to preventing current MPs standing again as Labour MPs at the next election or whether he was describing the current system, but either way it's more of driving a wedge between different parts of the party, setting the members against MPs, setting MPs against members.
"One of the responsibilities of leadership is actually to bring people together, not to set people against each other.
"I think it is very unfortunate and another example of why really we need a new leadership rather than Jeremy Corbyn."
Mr Corbyn, who is fighting former shadow work and pensions secretary Owen Smith to hang on to the party's top job, is favourite to win the postal ballot of Labour's members - whose ranks he said had swelled to more than 500,000 - as well as the 183,000 people who signed up this week as registered supporters and the affiliated supporters in the unions.