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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Generally cloudy with fairly persistent rain through this evening.
A cloudy evening, with sporadic outbreaks of rain or drizzle. These will ease away overnight, with a few clear spells around dawn.
The House of Commons Petitions Committee has said they will keep an eye on the petition and are concerned about fraudulent signatures.