The announcement was followed by the leaking of an alleged Royal Mail presentation on the strike, which describes the dispute as an "enabler" of its aim to modernise the service.
It also states the dispute could result in it reducing the role of the CWU.
Royal Mail issued a denial of the document, which was leaked to the BBC's Newsnight programme, disowning the strategies discussed in the presentation and ruling out the possibility that it originated at the company.
The CWU announcement comes as consultancies advise brands to switch to alternative mail suppliers such as TNT Post, UK Mail, Citipost, DHL and UPS.
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