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The union said any subsequent reduction in visitors could spark further cutbacks, leading to the Duxford site shutting during the winter and families being charged to bring in their children, who currently go free.
Last year IWM began reviewing its operations to reduce spending by £4m a year because of reductions in the grants it receives.
The cuts came at a time when it was spending £40m towards the refurbishment of its main London museum in time for the centenary of the outbreak of World War One.
Also:
http://www.bbc.com/news/uk-england-london-29966987
8 November 2014 Last updated at 14:13 GMT
Plans to close the Imperial War Museum (IWM) library in London have been opposed in a petition signed by 1,500 people.
The Prospect union said a £4m cut in annual government funding left the institution facing the closure of the library and the loss of 80 jobs.
An IWM spokesman said the plans were in response to the need to "reduce costs" and "respond to challenges and opportunities".
The decision comes as the museum marks the centenary of World War One.
The proposal also recommends the closure of the Explore History facility, which allows the public to find out about objects not on display, and attracted 55,000 visitors in 2013.