Union officials said the Government was planning to attack hours, holidays, sick pay and flexible arrangements under a review of working conditions announced last year.
PCS general secretary Mark Serwotka said the lives of civil and public servants were being made more difficult by cuts.
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Extracts released by 10 Downing Street suggest Mrs May will say the UK will not settle for a "half-in, half-out" policy.
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In her Brexit speech on Tuesday, the Prime Minister is set to urge the EU to forge a fair trade deal or risk commercial war.