A total of 37 social security offices and jobcentres across the UK will close in the first wave of government plans to shed 104,000 civil service jobs.
The news prompted renewed calls for PCS members to vote to strike
Up to 2,000 jobs are under threat, according to Public and Commercial Services Union boss Mark Serwotka.
He said: "This is the first phase in the decimation of services to the vulnerable."
Earlier this week Mr Serwotka announced his members would be balloted for a one day strike on 5 November.
Mr Serwotka told a news conference on the final day of the TUC conference in Brighton that the closures will see offices shut down in "some of the poorest communities in Britain hitting the long-term unemployed and some of the most vulnerable in society".
"The government said that their so called efficiency drive was about putting more resources to the frontline yet today we see essential frontline services axed with no real thought of the impact on the communities they serve," he said.
"The government's agenda is exposed for what it is - it is not about how things can be done more efficiently or better, it is about crude cost cutting."