The Queen's staff at Windsor Castle are set to take industrial action over what they say is low pay.
Workers at the royal residence are being balloted on action over unpaid "extra duties" such as giving tours and acting as interpreters for visitors.
Currently, staff are expected to perform these additional duties out of "goodwill", the Public and Commercial Services (PCS) union said.
The Royal Collection Trust said staff volunteered for these roles.
PCS said "low paid" staff do not receive additional pay for providing extra services to castle visitors - even though tourists are charged for these.
If the 120 union members at the castle vote in favour of non-strike industrial action, it would see a "withdrawal of... goodwill", and would have "a significant impact on the services provided to visitors", the union said.
A Royal Collection Trust spokeswoman said: "Warden staff are offered voluntary opportunities to receive training and develop skills to lead guided tours for visitors as part of their working day and to administer first aid, as well as to use their language skills.
"These are not compulsory aspects of their role, and it is the choice of the individual whether they wish to take part."
According to the union, a pay offer for 2014 "was only narrowly accepted by staff" on the understanding they would receive additional allowances this year for extra services, but this has not happened.
PCS general secretary Mark Serwotka said: "These workers are loyal to their employer and absolutely committed to ensuring visitors are given the royal treatment.
"It is scandalous that staff are so appallingly-paid and expected to do work for free that brings in money for the royal family."
The castle does not "anticipate any interruption to the running of tours for visitors".