The Government is planning to close a number of Job Centres under reforms to deal with "under-used" buildings, according to the Public and Commercial Services (PCS) union.
Union officials said more than one in 10 Job Centres in England, Wales and Scotland will shut putting thousands of staff jobs at risk.
The Department for Work and Pensions (DWP) said it expected the employees in branches set to close, to move to other sites.
The DWP added that four out of five claims for Jobseeker's Allowance and 99% of applicants for Universal Credit submit their claims online.
Job Centres and benefit centres are covered by old building contracts which are coming up for renewal.
The proposals include merging 78 smaller Job Centre Plus offices in urban areas with larger ones nearby, co-locating around 50 offices with local authorities or other community services and closing 27 back office buildings.
More than 700 existing offices will be retained.
The Government said that any redundancies would be "very small."