The Queen is facing the prospect of members of the Royal household going on strike as staff are set to vote on industrial action.
Workers at Windsor Castle are to be balloted on strike action in a dispute over pay.
The dispute affects around 120 union members who work at the castle's visitor services, including guides and kitchen staff.
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."
A spokesman for the Royal Collection Trust 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. Wardens at Windsor Castle are paid above market median based upon the regional living wage and receive a range of benefits, including a 15% non-contributory pension and a free lunch.
"Royal Collection Trust continues to award wardens an annual performance-related pay increase of up to 2.5 per cent, in addition to the cost of living increase (in line with Treasury guidelines), as well as one-off payments to those who have reached the top of their pay scale.
"We don't anticipate any interruption to the running of tours for visitors to the castle."
