PCS claims that changes to working practices would mean reduced staffing, leaving some gallery attendants on their own in charge of large collections of artwork, and that this would become “the norm”.
The union said some staff feared taking “comfort breaks” over being held responsible for a room or floor closure, while some front-of-house staff had been approached to cover security shifts without proper training.
It also said the move would make it “increasingly difficult” to honour reasonable adjustments for staff with disabilities.
Ruby Gibson, PCS national officer for Scotland, accused NGS of failing to consult staff and refusing to engage with complaints.
She said: “It is ironic that while the Scottish government is attempting to save money for the public purse by cutting budgets and talking about worker-led reforms, employers are disregarding their workers’ views and wasting money on overtime.
“Any disruption to the galleries during this crucial time of the year will be because of their failure to consult.”