Mid Sussex MP Nicholas Soames has written to the Post Office to object to proposed branch closures.
Plans to shut offices in High Street, Lingfield Road and North End in East Grinstead and Queens Road and Sunte Avenue in Haywards Heath were announced last week.
Post Office Ltd says it can no longer afford to keep all 23 Mid Sussex branches open because there is not enough business to go round.
But in a letter to area manager Drew McBride, Mr Soames said: "I am very concerned to hear about these proposals, which will have serious implications for my constituents.
"A huge house-building programme has been imposed by the Government in West Sussex and Mid Sussex is obliged to accommodate 10,000 homes between 2001 and 2016.
"West Sussex also has one of the highest elderly populations and the elderly rely heavily on the services provided.
"I'm afraid that for this reason I question the rationale behind the Post Office's thinking."
Representatives from the Post Office are due to meet with Mid Sussex District Council to discuss the plans today although the meeting is not open to the public.
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