Party chairman Ian Lavery, who is weighing up a leadership bid, issues the alert in an exclusive article for the Mirror.
The MP for Wansbeck, Northumberland, blames the poll defeat on would-be rivals hampering the campaign by demanding a second Brexit referendum, fuelling party divisions.
The key Corbyn ally says: “ Labour lost because of our Brexit position, infighting, a collapse of industry in our communities and a lack of trust in our ability to deliver.”
Shadow Foreign Secretary Emily Thornberry and Shadow Treasury Minister Clive Lewis have entered the leadership race, with Shadow Justice Secretary Richard Burgon joining last night.
Shadow Brexit Secretary Sir Keir Starmer, Shadow Business Secretary Rebecca Long-Bailey and former Shadow Energy Secretary Lisa Nandy are expected to follow suit.
Mr Lavery warns: “For Labour to win again we need to understand why we lost and to rebuild trust and power in our communities.
“We don’t have the divine right to exist as a party – we need to earn it.”
Meanwhile figures show that Labour’s female leadership candidates ask more parliamentary questions than their male counterparts.
Ms Thornberry leads the way with 1,451 since 2015.