Just 35% (-12) of Labour members think the party will win the next general election in 2020 if Jeremy Corbyn remains in charge, a poll from Election Data and YouGov released on Monday (6 March) showed.
The survey, of more than 1,000 Labour members between 27 February and 3 March, also found that 56% (-15%) of respondents thought it unlikely that the party would defeat the Conservatives.
The polling comes just weeks after Labour's historic defeat to the Tories at the Copeland by-election in February. It was the first time since 1982 that the opposition had lost to the government in a by-election.
Corbyn refused to quit in the wake of the result and hailed Labour's victory at the Stoke by-election, which was held on the same day, as a victory.
"Labour's victory in Stoke is a decisive rejection of Ukip's politics of division and dishonesty. But our message was not enough to win through in Copeland," the left-winger said.
"In both campaigns, Labour listened to thousands of voters on the doorstep. Both constituencies, like so many in Britain, have been let down by the political establishment.
"To win power to rebuild and transform Britain, Labour will go further to reconnect with voters, and break with the failed political consensus."
But the new polling figures from Election Data showed that 41% of Labour members thought Corbyn was to blame for the defeat, while the media ranked second at 34% and Tony Blair's pro-EU, anti-Corbyn speech the week before coming in at third 22%.
However, the survey was not all bad news for Corbyn since Labour members backed him (34%) as the "most credible" future prime minister over Home Affairs Select Committee chair Yvette Cooper (26%) and Shadow Brexit Secretary Sir Keir Starmer (26%).
Elsewhere, the polling found that two thirds (68%) of the more than 500,000 Labour members back a second EU referendum, a policy which failed Labour leadership candidate Owen Smith called for. A spokesman for Corbyn had not responded to a request for comment at the time of publication.