The general secretary of the University and College Union (UCU) has launched an attack on the "factionalism" that she says has exposed the union to divide-and-rule tactics.
UCU Left, which is dominated by the Socialist Workers Party, was the only union group mentioned by name in Sally Hunt's speech to the union's annual congress last weekend, which argued that too many of UCU's resources were being diverted from helping ordinary members to service a political agenda.
Ms Hunt said: "I think we also need to ask ourselves whether the factionalism that has recently emerged in UCU discourages members from wanting to get more involved.
"We all know to our cost that in FE and HE, the creation by vice- chancellors and principals of rival interest groups has diluted the sector's message and allowed Government to divide and rule. UCU must avoid making the same mistake.
"I don't care whether it's UCU Left, right or centre, or UCU I-haven't- made-my-mind-up. Or whether it's the Socialist Workers Party, the Green party, the Conservative party, Labour party or the Monster Raving Loony party. I would say the Lib Dems, but there aren't any left.
"Any group that seeks to assert its own political agenda in place of that of our members should think again."
She said the union needed to tackle the bloated size of its 68-member national executive committee, which she said was as large as Unison's despite UCU as a whole being less than a tenth of the size of the public sector union.
Many of the positions on the committee were uncontested as a result, which she said showed the need to re-engage ordinary members.
Ms Hunt said the UCU could save pound;150,000 a year in administrative expenses if the size of the committee was reduced, which could be reinvested in better legal support for members and help for branches.
But her speech drew some opposition, with one member receiving applause later in the conference as he called on the union to stop looking for "enemies inside this hall".
UCU Left is expected to field a candidate against Ms Hunt when she goes up for re-election later this year. A similar call earlier this year to "reclaim the union" from a minority of national executive committee members was met by a counter-accusation of factionalism.
"We should stop attempting to knock lumps out of each other, and should concentrate our energies on winning large turnouts in all ballots, large majorities in favour of action, and maximum member involvement in the action that follows," UCU Left said.