Post Office managers will stage their first industrial action in 34 years in a dispute over pay.
The Unite union, which represents 900 managers, said its members will ban overtime and work to rule for 24 hours.
The union accused the Post Office of "dragging its feet" for 16 months over a pay deal and warned of further industrial action if the dispute is not resolved.
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Political parties across the country suspended their General Election campaigns following the Manchester Arena terror attack on Monday.
Investigators believe Mr Kushner has significant information relevant to their inquiry, officials told NBC News.
The men were detained as part of an investigation by MI5 and the Metropolitan Police's Counter Terrorism Command.