Mandelson: Corbyn must demonstrate 'unelectability'
The former Cabinet secretary was among the most vocal critics of Mr Corbyn's leadership bid, warning that his victory would be a "mortal threat" to Labour.
Now, in a private paper seen by the Guardian he warns associates that by electing the veteran leftwinger, Labour members are "putting two fingers up" to the public.
However he warns against a premature coup against Mr Corbyn, saying: "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 will face another major test of his nascent leadership when he makes his keynote speech to Labour conference in Brighton next week.
He has already risked a flashpoint with members of his shadow cabinet by declaring that he intends to wholly oppose the Tories' Welfare Reform Bill, including the household benefit cap - a move publicly opposed by figures such as Shadow Work and Pensions Secretary Owen Smith.