A postman, based in London, hailed the union’s work, and warned the strikes could “get very serious”.
He told Express.co.uk: “The unions will not back down now and have been brilliant so far.
“They’re trying to protect the working rights of the next generation of workers, just as much as they are for us now.
“I’m quite sure the unions will win in the end.”
A Royal Mail spokesperson said the company invited the CWU to “enter into talks” to “find a resolution to our dispute on change and pay” on September 22.
They added: “This evening, rather than responding to our offer of Acas talks, the CWU announced further damaging industrial action, once again taking the path of prolonging disruption over resolution.