Plans to bulldoze the Grade II-listed Edwardian Wing of the former Jessop Hospital could be the subject of a legal challenge.
Sheffield Council controversially approved the proposals by Sheffield University, which owns the site and wants to use it for an £80 million engineering block, last December.
There was strong opposition from groups including Hallamshire Historic Buildings Society and Sheffield Victorian Society.
Now another conservation organisation, Save Britain’s Heritage, has sent a ‘pre-action letter’ to the council saying it had ‘misinterpreted’ national planning policy.
Clem Cecil, director of SBH, said she believed the council had failed to meet a requirement of the National Planning Policy Framework that the public benefit of knocking down a listed building must ‘outweigh the harm’.
She said: “We are asking the council to reconsider its decision and, if they refuse, it could go to judicial review.
“We believe the council has misinterpreted national planning policy.
“It deserves to be reviewed in court if national law has been misinterpreted or misapplied.”
A Sheffield Council spokesman said: “Sheffield Council has taken legal advice and has been advised not to comment. But it will defend its position if raised in court.”
The university hopes to open its new building in 2016.