Senior members of the University and College Union have accused the Socialist Workers Party and a lecturers' Left group of seeking to "take over" the union while using its members and resources as "cannon fodder".
In an email to branch officials, the union's treasurer and two members of the national executive committee (NEC) argue that the actions of "SWP/UCU Left" have "undermined our credibility with our members and strengthened the hand of the employers".
The union, which has about 65,000 members in the academy, is holding five strike ballots across further and higher education this month over jobs, pay and pensions.
Supporters of the UCU Left, an increasingly influential force on the NEC, say the email is counterproductive at a crucial time for the union.
They criticise "red scare" tactics, saying UCU Left members are independent and come from a range of political backgrounds.
The email comes after Sally Hunt, the UCU's general secretary, warned the NEC that union strategy was being "directed by bodies outside UCU rather than our own members".
Divisions have hardened after the UCU Left successfully pressed for the union to support a protest by left-wing student groups in London on 29 January, against fees and cuts.
This angered opponents - including Ms Hunt - who noted the violence of previous marches and warned of damage to the UCU's relationship with the National Union of Students.
In the email, the three members of the "independent broad left group" of the NEC - Alan Carr, the union treasurer, John McCormack and Angela Roger - appeal for support for a Reclaim the Union campaign.
They criticise "the sectarian behaviour of the Socialist Workers Party, which dominates and controls...'UCU Left'", and say the groups "are seeking to take over our union".
The email blames them for the "much ridiculed 8 per cent pay claim in higher education in 2009".
Previous criticisms of the UCU Left have focused on the proposed boycott of Israeli universities.
"The SWP/UCU Left seems determined to use members and the union's resources as cannon fodder, and the only winners from this folly will be the employers and the government," they say.
"While we seek the support of local NUS students for our own disputes, the NEC has created an alliance with student organisations (that) publicly attack NUS."
UCU Left supporters said that the three signatories supported earlier versions of motions backing the London protest and asked why they "went along" with such motions if they believed there was an "SWP plot".
An SWP spokesman said it was "hardly a secret" that party members are active in the UCU, "but the idea that there is an attempt at a 'palace coup' by a group of socialists to take over the union is frankly crazy".