Visitors to the Imperial War Museum and HMS Belfast will in future be greeted by attendants employed by a private security firm rather than in-house museum staff.
"After a very thorough evaluation of our options, it has been decided that IWM will move to an outsourced provision for visitor services and security from 1 April 2014," said a museum spokeswoman.
"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."
The chosen contractor is the Shield Group, which describes itself as "one of the UK's leading providers of total security solutions".
The firm's local contracts include the South Bank Patrol warden service.
The decision has been criticised by the PCS union which says it fears that jobs will be cut and that the move is driven by financial considerations.
"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," said PCS general secretary Mark Serwotka.
"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."
An IWM spokeswoman said: "IWM would not take any action that would jeopardise the museums, the quality of the visitor experience, the rights of staff or the First World War Centenary commemorations.
"The greatest priority for both IWM and the Shield Group now will be to meet with Staff and PCS in the New Year to ensure everyone is fully supported throughout this transfer.
"We will work hard to ensure the excellent knowledge our visitor services and security staff bring to IWM remains and develops within the teams and continues to benefit all who visit our museums."