Visitor and security services at Imperial War Museums are to be privatised, affecting staff in London, Manchester and Cambridgeshire.
The move will affect more than 100 staff at the war museums in London and Manchester, HMS Belfast and Churchill War Rooms in London, and Duxford air museum in Cambridgeshire.
The successful bidder for the contract is the Shield Group, with staff employment conditions protected by employment regulations.
A statement said: "This decision was based on ensuring that our long-term quality needs can be satisfied by a provider who delivers optimum value for money and also excellent career opportunities for staff."
Mark Serwotka, general secretary of the Public and Commercial Services union, said: "As the eyes of the world look to the Imperial War Museum to mark the centenary of the First World War, it is taking a major gamble with its reputation.
"We believe that gifting this work to a profit-making company will lead to jobs and standards being cut, and risks a knowledge drain from these highly-prized visitor attractions."