Peter Mandelson has reportedly advised Labour critics of Jeremy Corbyn against forcing efforts to replace him until voters judge him unelectable.
The peer said opponents of the new Labour leader were "in for a long haul" during which the atmosphere in the party will become "increasingly acrimonious at branch and constituency levels", according to The Guardian.
And the Labour former minister, who was a key figure behind New Labour, said moderates in the party at grassroots level needed to be given hope there was a "way out of our predicament and that Labour does have a future".
Lord Mandleson made the latest comments last week in a private paper circulated to political associates, The Guardian said.
He wrote: "In choosing Corbyn instead of Ed Miliband, the general public now feel we are just putting two fingers up to them, exchanging one loser for an even worse one. We cannot be elected with Corbyn as leader.
"Nobody will replace him, though, until he demonstrates to the party his unelectability at the polls.
"In this sense, the public will decide Labour's future and it would be wrong to try and force this issue from within before the public have moved to a clear verdict."
Lord Mandelson previously voiced concerns about Mr Corbyn's potential impact on the party before the Islington North MP swept to victory after attracting a massive 59.5% of votes.