A total of 97.6% of those who voted backed strikes, on a turnout of 77%, in protest at a 2% pay offer, the Communication Workers Union (CWU) said.
Royal Mail chief executive Simon Thompson said: “I am ready to talk about pay and change at any time. But it has to be both.
“When we previously spoke to the unions we said there needed to be an improvement in productivity, but it has not gone forwards but backwards.
“Without change this will continue, and obviously we do have concerns over the impact from yesterday’s CWU announcement.”
Royal Mail also tweeted: “We’re disappointed that members of CWU have voted in favour of industrial action. This ballot result does not necessarily mean there will be industrial action. We will continue to seek an agreement.”
In response to the tweet, CWU’s communications team hit back with a dazzling tweet, writing: “Dry your eyes mate.”