The Government is to close its Central Office of Information which union leaders warned will threaten the loss of 400 jobs.
Prospect said it was "shocked and devastated" by the decision to abolish the information department, which was set up in 1946.
Ministers said the closure was part of a reform of Government communications following spending controls on advertising and marketing last year.
Prospect said there had been no consultation or warning to staff about the likely redundancies, adding that employees at the COI's London head office would bear the brunt of the cuts.
The Cabinet Office said changes leading to the closure of the COI would improve the effectiveness and efficiency of Government communications.
Francis Maude, minister for the Cabinet Office, said: "This Government has slashed unnecessary spending on communications. These important and significant changes to Government communications structures are designed to reflect this and to save more money by cutting bureaucracy and reducing duplication.
"This does not mean the end of vital and cost-effective marketing campaigns, such as those campaigns that save people's lives. However, it does mean that communications spending in the future will never again get out of hand and instead will be more transparent, better co-ordinated and less bureaucratic."
The Government said spending on advertising and marketing had been cut by 68% to £168 million in the past year, with departments reducing the number of in-house communications staff by a quarter and their budgets by half.
Campaigns such as health and recruitment to the armed forces will continue, the Government said.
Prospect general secretary Paul Noon said: "Staff are shocked and devastated. This has come completely out of the blue. Across government, ministers are centralising finance, HR and procurement in order to save money and cut duplication. At COI, a shared service that has worked well and is respected by the industry in which it operates is about to be chopped into little pieces. It makes no sense at all."