About 110,000 Royal Mail staff went on strike on Thursday to demand better pay and working conditions.
It was the first of 19 days of strikes planned to coincide with the peak holiday shopping season, including Black Friday and Cyber Monday.
In a statement posted to its website on Friday, the union said it was demanding a government inquiry into the “gross mismanagement” of Royal Mail.
Royal Mail has said that most letters will not be delivered on strike days, and customers should expect delays to parcel deliveries.
It would deliver as many tracked parcels and those tagged for fast-track delivery as possible on strike days, the company added.