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."
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The Formula One test driver María de Villota, who lost the sight in one eye after a crash in Cambridgeshire, has been found dead in a hotel room. Police are calling it a "natural death".
Last year, the Spanish driver crashed at Duxford Aerodrome, while test driving for the Marussia F-1 team.
The 33-year old's body was today discovered in Spain, where she was promoting her autobiography.
Click below to watch a report from ITV News Anglia's Luke Farrington
María de Villota's manager, Mark Blundell, believes that the Spaniard would have been "the next female Formula One driver".
De Villota was found dead in a hotel room in Seville earlier this morning.
The 33-year-old's racing career was cut tragically short when she lost an eye in an accident at Cambridgeshire's Duxford Aerodrome last year.
However, Blundell believes that De Villota had the talent to become the first female driver to enter the Formula One World Championship since Giovanna Amati back in 1992.
Formula One teams and circuits have been paying tribute to former Formula One test driver María de Villota after the 33-year-old was found dead in a hotel room in the Spanish city of Seville.
Norfolk-based Lotus Cars and Silverstone Circuit in Northamptonshire were among those offering their condolences.
We are shocked and deeply saddened to hear of the death of María de Villota. Our sincere condolences to her family. http://t.co/3lH7IVJPrb
We are so very sad to hear about the passing of Maria de Villota this morning, our thoughts are with her family and friends
IWM Duxford have released a statement after learning of the death of former Formula One test driver María de Villota.
The Spaniard was found dead in a hotel room in Seville this morning.
De Villota was previously involved in a freak accident at Duxford Aerodrome in Cambridgeshire that resulted in her losing her right eye.
"Management and staff at IWM Duxford are truly saddened to hear about the death of María de Villota.
Our sincere condolences go to her family, friends and colleagues at this difficult time."
Former Formula One test driver Maria de Villota has died at the age of 33.
On July 3 last year, whilst testing for Marussia, De Villota was involved in a freak accident at Duxford Aerodrome in Cambridgeshire that resulted in her losing her right eye.
Despite spending a month in hospital, De Villota rebuilt her life and became part of the FIA's Women In Motorsport commission.
Spaniard De Villota was the daughter of former F1 driver Emilio, and competed in F3, GTs, touring cars and the Superleague Formula before joining up with Marussia.
It is understood De Villota was on a tour promoting her autobiography at the time of her death, with reports in Spain stating she was found in a hotel room in Seville.
A statement on De Villota's Facebook page, signed by her family, read: "Dear friends: Maria has left us.
"She had to go to heaven like all angels. We are thankful to God for the extra year and a half that he left her with us."
The Imperial War Museum at Duxford in Cambridgeshire is preparing for its last airshow of the season.
The event on Sunday marks the 40th anniversary of the first air display which was held at the museum in 1973.