Unite members will not undertake any overtime, will refuse to work any additional hours or do work not proper to their role in the organisation.
They will also refuse to travel outside of their working time, nor will they work in a location other than their normal place of employment.
This is an important day for Unite members in the Post Office.
They are keen to impress upon their employer their dissatisfaction with the failure to reach a pay agreement with the union and see this an opportunity to protest.
More top news
The singer, 31, said on Twitter that she and her partner Jonathan Powell were grieving their loss.
A fine evening ahead to the south with some late sunshine in places.
Prince Charles and the Duchess of Cornwall have visited Manchester Arena, the site of a suicide bombing in May which left 22 people dead.