The union, which represents professional gardeners working for the heritage body among others, is publicising the issue ahead of the annual general meeting in Swindon on 8 November.
The union says that low pay within the organisation has left many of the trust's 12,000 employees struggling below the poverty line and it is calling on the National Trust’s board of trustees to "take a stand" against low pay.
Prospect negotiations officer John Stevenson said: "Downton Abbey is alive and kicking in the National Trust. The living standards of employees have plummeted in real terms despite the good financial performance of the organisation.
"These are the guardians of our national heritage, yet they are left to struggle on with wages from a bygone era. Aside from the fact it is morally right, if we learn anything from history, it should be that getting money into people’s pockets is essential to securing a strong economic recovery."