Labour MPs must wait for Jeremy Corbyn to prove his lack of popularity before they replace him as leader, Lord Mandelson has said.
The former Labour Business Secretary told his political allies they must prepare for the "long haul" under Mr Corbyn and must wait until the public has delivered a "clear verdict" on his leadership.
He also said that by "exchanging one lose for an even worse one" by replacing Ed Miliband with Mr Corbyn was akin to "putting two fingers up to" voters.
His comments came in a private paper circulated to his political allies last week, seen by the Guardian.
During the leadership election Lord Mandelson did his utmost to persuade those eligible to vote against opting for Mr Corbyn, warning he would put Labour in "mortal danger".
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But writing privately to colleagues, he said: "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."
Mr Corbyn's team has not commented on Lord Mandelson's latest remarks.
Only around 20 Labour MPs actually voted for Mr Corbyn in the leadership race, despite him securing the nomination of 36 MPs to put him on the ballot for the contest.
Those determined to oust Mr Corbyn before the 2020 election are discussing in private the best time to make their move, with next May's elections for London Mayor, the Scottish Parliament and Welsh Assembly seen as a decisive point in judging Mr Corbyn's leadership.
Some in the parliamentary party believe a poor performance in these elections will finally prove to Corbyn supporters that Labour has no chance of winning in 2020 without a more moderate leader.
