Staff working for the Royal Collection Trust at Windsor Castle have voted in favour of industrial action in a dispute over pay.
The RCT is a registered charity that looks after the royal collection of artwork and artefacts and promotes access to it.
In a ballot of 76 of the 108 wardens at the royal property in Berkshire, who help to protect and maintain the Royal Collection, 64 voted in favour of industrial action.
The Public and Commercial Services Union, which represents wardens at the castle, said their pay started at £14,400 a year and non-strike industrial action could begin by the end of the month. It said this could include the withdrawal by wardens from voluntary activities such as interpretation and tours of the castle, for which visitors pay extra.
The PCS said the wardens "only narrowly accepted an unsatisfactory pay offer last year on the understanding that additional allowances for paid-for tours and other skills would be considered this year".
Mark Serwotka, general secretary of the PCS, said of the wardens: "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."
A spokeswoman for the RCT said in a statement that although the outcome of the ballot was disappointing, it would have no effect on services to visitors to the castle.
She said that since last year the RCT had been "exploring ways to achieve an agreed level of pay for all warden staff".
She said: "Conversations that are part of the annual pay review process are still ongoing, and an offer to expand the salary scale for a warden, starting at the regional living wage of £14,695 for new joiners (based on an average 36-hour working week), has been put to PCS and other unions."
She said the activities that the wardens would cease to carry out as part of the industrial action, such as using their language and first-aid skills, had always been the choice of each individual warden.