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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Some 66 people have been killed after the plane they were travelling on crashed in southern Iran.
The letter calls for an end to harassment, abuse, and impunity in a world "ripe for change".
Police say that anyone who sees Whitehouse should not approach him, but instead call them on 999 and quote incident 23 of 18 February.