Delegates to the UCU union's annual congress defied the leadership on Wednesday by voting in favour of debating two motions critical of general secretary Sally Hunt.
Workers employed by Clece Care Services, which provides transport assistance at Luton airport to passengers with reduced mobility, struck on Monday and Tuesday.
Hundreds of the lowest-paid hospital workers took to the picket lines across Wigan on Wednesday. It marked the first day of a 48-hour strike by Unison and Unite union members against outsourcing.