Unite's Len McCluskey says the establishment are too scared of Jeremy Corbyn to call an election.
The head of the UK's largest union said the Tories would have risked calling a poll if Ed Miliband, Tony Blair or Gordon Brown were in charge.
Speaking at Labour's music and politics festival the union firebrand also hit out at Labour MPs on the 'right wing'.
He accused MPs and Labour staff of wanting Corbyn to fail but said they were "slapped in the face by millions of people" by the general election in 2017.
Crowds gathered in the Solidarity Tent to discuss a range of political issues with questions on issues from internal Labour Party politics to the National Health Service and the lack of representation of black and ethnic minority people in politics.
Quoting Shakespeare, Mr McCluskey told the so-called doom and gloomers within the Labour Party: “If you have no stomach in this fight, depart the battlefield".
He told an audience at a People's Question Time event at Labour Live. "If the Labour Party was being run by Ed Miliband or by Gordon Brown or by Tony Blair then we would have had an election by now because the establishment wouldn’t be afraid of what might happen if Labour got into power but they’re scared stiff of Corbyn and that’s why we’ve got to focus on getting him into Number Ten.”
Concerns that the party could be embarrassed by low turn-out at the festival saw ticket prices slashed from £35 to £10 in recent weeks.
But when the Labour leader took to the stage, thousands of supporters filled North London's White Hart lane recreation ground chanting the now familiar "Oh, Jeremy Corbyn" to the tune of Seven Nation Army by the White Stripes.
Mr Corbyn congratulated attendees on the party's showing in last year's election, stating: "My only sadness is that we didn't quite win that election."
The Labour leader branded the Tories "the party of the rich for the rich".
Mr Corbyn said his message for the Tories was: "You crept back into office a year ago. You started bunging money around in order to stay in office.
"For goodness sake, give up, move on, move over and let the people take over."