Keir Starmer risks being “dumped into the dustbin of history”, Len McCluskey has warned.
The Unite union general secretary said voters “don’t understand” what Labour stands for anymore.
In an interview with Times Radio on Sunday, McCluskey, who was a close ally of Jeremy Corbyn, said Starmer was on course to “destroy the unity of the party”.
McCluskey also said the selection process led to the selection of Paul Williams as Labour’s candidate for the Hartlepool by-election “beggars belief”.
Williams chosen to stand in the seat after he was the only person placed on the shortlist of candidates.
In the interview, McCluskey said: “Keir needs to start telling people what he is and what Labour are. People don’t know at the moment.
“People knew where Jeremy Corbyn was coming from long before any elections.
“People knew where Tony Blair was coming from long before any elections.
“At the moment we are suffering because people don’t understand what Keir Starmer stands for or what Labour stands for. And that’s what he has to do.
“Stick to the radical nature of the policies he stood on and win back the red wall seats.
“If he continues to attack the democracy in the left of the party he’ll destroy the unity of the party and the reality will be he’ll be dumped into the dustbin of history.”
Speaking about the upcoming Hartlepool by-election, McCluskey said: “If that’s Keir’s definition of democracy well it’s certainly not mine. It’s not even a pretence any more.
“A shortlist - actually it was a longlist - of one man. It beggars belief. But at the moment in terms of the internal democracy within our party. Nothing is surprising.
“I regret the fact that a proper process wouldn’t have been gone through.”
The by-election in the so-called red wall seat was triggered after incumbent Labour MP Mike Hill resigned this week amid sexual harassment allegations.
The contest will be seen a key test for Starmer’s leadership, one year after he succeeded Corbyn at the top of the party.
Boris Johnson made advances into traditional Labour territory in the North of England, Midlands and north Wales during the general election in December 2019, in which he secured a Conservative majority victory.
One of the seats taken as part of the host of red-to-blue turnovers in the North East was Williams’ former Stockton South seat.
Williams has apologised after a Tweet of his from 2011 was unearthed in which he asked his followers: “Do you have a favourite Tory milf?”
Shami Chakrabarti, the former shadow attorney general under Corbyn, has said the “unacceptable misogynistic” language means Williams should be dumped as the candidate.
But speaking to the Andrew Marr Show on Sunday, Lisa Nandy, the shadow foreign secretary, agreed while the language was “completely and utterly unacceptable”, as Williams had apologised he should not have to step down.