Beefeaters at the Tower of London are among those being balloted over possible strike action amid a row over planned changes to their pension scheme.
Members of the GMB union employed by Historic Royal Palaces, the charity that manages unoccupied royal sites including Hampton Court Palace, Kensington Palace State Apartments and Kew Palace, will begin voting tomorrow.
Employees are upset that HRP, which was spun out from the public sector as a royal charter body in 1998, plans to change their pension scheme from a defined-benefit version to a defined-contribution scheme.
The GMB said that for those affected the move amounted to "a pay cut for the rest of their lives".
The union said the charity promised at the time of the transfer that employees’ pensions would not be changed.
The charity, which employs about 1,200 people, said the scheme had become unsustainable and posed too great a risk for the organisation.
About 50 of the approximately 200 GMB members working at HRP will be balloted, with the vote open until 17 December.
If members vote for strike action the first date that has been pencilled in is 8 January.
Mick Ainsley, regional officer at the GMB, said: "Pensions really matter to our members and, in the day-to-day contacts we have with them across HRP, the importance to them of continuing to be able to build the pension they were promised when they were transferred to HRP from the civil service is emphasised to us over and over again.
"They see their defined-benefit pension entitlement as a critical element of the employment package and quite rightly put a great value on it.
"HRP is just jumping on the bandwagon as it sees other companies ending good defined-benefit schemes, preferring instead to let the taxpayer pick up the shortfall in the years to come.
"GMB members have made it very clear that they want more negotiations and are prepared to talk. Strike action is the last thing they want to do, but HRP has left them with nowhere else to go."
John Barnes, chief executive of Historic Royal Palaces, said the planned changes affected 11 per cent of staff.
"The scheme has become financially unsustainable and the rising costs pose too great a risk for Historic Royal Palaces," he said.
"We need to act in the best interests of the charity and of the majority of our staff, who will all benefit from a 2 per cent increase in employer contributions to their pensions from April 2019 as part of the changes.
"This is not a decision we have taken lightly, because we value the contribution of our staff immensely. We have consulted extensively with the staff affected over the course of the past year, as a result of which they will be receiving compensation and transition arrangements that are among the best in the market. They will be joining the same competitive defined-contribution scheme as their colleagues."