Workers in hundreds of Crown Post Offices are set to stage their 12th round of industrial action since Easter.
Members of the Communication Workers Union in England, Wales, Northern Ireland and most of Scotland will walk out on Monday, followed by a strike in a handful of branches in Scotland on Tuesday.
Staff are also taking other forms of action including a sales ban on financial products and services.
The union is opposed to plans to franchise or close 75 Crown offices, the larger sites usually found on high streets.
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Some customers are suspected of passing the expensive items off as carrots at self-service tills by switching labels.
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