Hundreds of Crown Post Office managers will go on strike today in a row over pay.
Unite warned that the action will hit services at the larger branches, usually sited on high streets.
The union, which represents 900 managers, said its members have not received a pay rise since June 2011 and accused the Post Office of "dragging its feet."
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Temperatures will be on the rise in the next few days, with highs of 25C in the sunshine by Thursday, but do not expect clear blue skies.