Post Office workers have walked out in a fresh dispute over pay, potentially resulting in 114 branches being unable to open.
Members of the Communication Workers Union (CWU) at Crown Post Offices - the larger branches often found on high streets - went on strike on Saturday.
Union members in administration and supply chain networks will strike on Monday.
The industrial action means there will be no cash collections or deliveries to post offices tomorrow, the union warned.
However the Post Office said it expected the "vast majority" of its branches to be unaffected by the strike.
Saturday marks the the second bout of industrial action since the dispute flared and the union is not ruling out further strikes.
The union said Post Office management are insisting on a pay freeze for 2021-22 and a "marginally improved" pay offer for 2022.
CWU assistant secretary Andy Furey said: "Our members do not want to be in this situation, but they won't accept humiliation either.
"Our members worked hard to deliver an excellent annual profit for the company - affordability isn't management's problem here.
"Frankly, their degrading excuse for an offer has only hardened members, who won't tolerate a collapse in their living standards.
"We thank the public for their understanding and support, and we urge the Post Office to get round the table and thrash out a real settlement that treats key worker heroes with the respect they deserve."
Around 3,500 union members are involved in the dispute.
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A Post Office spokesperson said: "We want to assure our customers that the vast majority of our branches are unaffected by CWU strike action on Saturday.
"There are 114 branches, typically in city centres, that are directly managed by the Post Office.
"Over two thirds would normally only be open between 9am and 12.30pm on a Saturday. We apologise to any customers who are inconvenienced by a closure."