Fire officials have written to every UK university warning there are "no excuses" for dangerous breaches of fire safety in student accommodation.
The Fire Brigade Union sent the letters after a photo was posted online of a fire exit at Marsden House in Leeds being tied shut with cable ties.
IQ Student, which manages the building, said it was "secured temporarily" due to concerns "about a faulty catch".
The union said obstructing fire exits "can, and has, cost lives in the past".
The warning comes as a fire exit at a university halls of residence in Nottingham was also fastened with cable ties.
A student living in The Glasshouse, in Union Road, took a photo of the double doors, which was then circulated on social media.
The student, who wishes to remain anonymous, said she believed it was the only fire exit on the lower ground floor of the building and the fastenings had subsequently been removed within hours of the image being posted online.
"In a very extreme scenario, if something were to happen then we wouldn't be able to get out. We'd be trapped," she said.
Hi @AboutHFS @TweetHFS I understand the red cable tie is regulated, however I would like to know about the black cable tie above it please?— Lyd (@Lydia_Caunce) September 30, 2020
This is The Glasshouse, Nottingham student accommodation fire exit.
Have you cable tied shut a fire exit?
Hope I’m wrong tbh. https://t.co/DF9Ga2o9sX pic.twitter.com/bnMdyWz2TO
Homes for Students, which runs the accommodation, said: "On Wednesday evening a student brought the photograph to our attention. This was dealt with immediately and we are investigating the individual circumstances but can confirm it was an isolated incident."
The cable ties on the fire exit at Marsden House were removed on Monday after the West Yorkshire Fire and Rescue Service were alerted by the photo.
Student Michael Snelson, 18, claimed the gate was secured to keep non-residents from entering the building.
In a letter, which has been backed by the University and Colleges Union, FBU General Secretary Matt Wrack said he had "major concerns" about the incident at Marsden House.
He warned all UK universities "that blocking, locking or impeding fire exits is both dangerous and illegal - to be clear, the pandemic is no excuse, nor mitigating factor for this sort of action".
Mr Wrack said safety was "absolutely paramount" and highlighted the Regulatory Reform Fire Safety Order (England and Wales, 2005), which states emergency doors must be easily opened and not locked or fastened.
He also said ministers needed to issue "clear guidance on health and safety on campus to all universities".
IQ Student said safety was "our highest priority" and it was continuing to investigate the incident.
It added: "We are undertaking additional training of the team working at the property, organising additional checks to all exits, and working with our employees and third party contractors to ensure their staff understand our protocols."