The Queen's staff at Windsor Castle are set to take industrial action in a row over pay.
Union members voted by 84 per cent for the action but stopped short of backing a strike.
The Public and Commercial Services union said wardens carry out extra duties such as giving tours of the castle to fee-paying visitors, without pay.
The 76 wardens involved in the dispute are employed by the Royal Collection Trust, working at the entrance to the castle, around the grounds and inside the palace.
PCS general secretary Mark Serwotka said: "These loyal workers are the public face of Windsor Castle and with this vote their message to their employer is loud and clear.
"Staff should be properly rewarded for their commitment to ensuring visitors from around the world can fully enjoy their time at the castle."
Windsor Castle attracts 1.1million visitors per year and is the largest and oldest occupied castle in the world.
