Jim Reeves, from the Communication Workers' Union, says the argument to close 70 Crown Post Offices is flawed because the business is profitable.
He spoke to ITV News Central outside the Kings Heath branch in Birmingham as staff went of strike this morning.
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Breezy and rather cloudy this evening and overnight.
Mainly dry to start with some clear spells but remaining windy and mild.
Bright and blustery conditions with a few scattered showers.