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The most widely used centres each help tens of thousands of taxpayers every year.
The closures will mean more than 2.5 million pensioners, vulnerable workers and tax credit claimants will lose a vital service, and put the jobs of 1,300 low paid, skilled workers at risk.
The impact on migrant workers in particular will be huge, as they make up one quarter of all users and often have complex tax queries.
The closures will also lead to more taxpayers with the wrong tax code, the union believes, and families getting into financial difficulties because they were not getting information about their tax credits.
HMRC's public consultation on the proposals last year was deeply flawed and overestimated the potential savings, the unions says.
In a separate move, HMRC also announced today a voluntary exit scheme for 500 more admin assistant grade staff across 70 offices, as part of wider plans to cut a further 10,000 jobs from the department by March 2016.
PCS general secretary Mark Serwotka said: "HMRC has failed to make the case for closing these offices that provide a lifeline for vulnerable taxpayers.
"These closures seriously undermine the government's claim it wants to ensure people pay their taxes, and it makes no economic sense to continue cutting jobs from the very department that collects the revenue that funds the public services we all rely on."
See the article on the PCS website here.