Beefeaters at the Tower of London are going on strike for the first time in 55 years in a row over their pensions.
Jewel house wardens and staff will picket at some of the UK's historic royal palaces including the Tower of London and Hampton Court Palace.
About 120 staff are being moved from final salary pensions onto an "inferior" model, the GMB union said.
"None of our sites will be closing as a result" said John Barnes, chief executive of Historic Royal Palaces.
The three-hour walkouts will also take place at Kensington Palace from 11:30 GMT on Friday, 28 December and 2 January by members of the Public and Commercial Services Union.
A separate strike by GMB workers will take place on 8 January.
Mr Barnes said: "We have already offered substantial compensation and transition arrangements to the 11% of our staff who are affected.
"The benefits they have already accrued will be fully protected, and they will be transferred onto the same competitive defined contribution scheme as the other 89% of their colleagues in April 2019."
The strike "will not change our decision", Mr Barnes added.
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The GMB said 91% of its members supported walking out after a turnout of 88%.
Michael Ainsley, GMB regional organiser, said: "Historic Royal Palaces are jumping on a bandwagon which is seeing employers engaging in a race to the bottom, ending good final salary schemes and replacing them with risky cheaper defined contribution schemes.
"GMB members still at this late stage are willing to talk to Historic Royal Palaces but the silence is deafening."