Up to 75 jobs could go at the University of Southampton as part of a restructure.
The university plans to merge eight faculties into five by August 2018.
It said between 50 and 75 posts would be lost in six subjects as part of plans to improve the university's ranking.
The University and College Union (UCU) said the "rushed and dangerous" proposals could undermine the quality of teaching.
Regional official Moray McAulay said: "You cannot deliver world-leading education by cutting staff."
She said the union would be considering action although a strike was "unlikely in the short term".
The university is challenging its 'bronze' ranking for degree teaching in a national survey in June.
Academic posts would be cut from December onwards in music, English, law, chemistry, tribology and social sciences, the union said.
The university said its plans to merge faculties would allow it "to run more efficiently and effectively whilst further strengthening research and interdisciplinary activity".
A spokesman said: "It's about... increasing our national and international rankings to secure a position in the top 10 in the UK and top 100 internationally. "
The university said it hoped to achieve the staff cuts through a voluntary severance scheme.