About 12,000 postal workers are on strike in protest at job cuts and poor working conditions.
Members of the Communication Workers Union (CWU) have walked out for 24 hours at centres in London, Plymouth, Edinburgh, Darlington, Bristol and Stoke.
The union is accusing the Royal Mail of cutting jobs and increasing workloads without agreement, at the same time as reducing services.
In an attempt to keep up with independent competitors, Royal Mail is modernising its delivery and sorting centres.
This involves the introduction of technology that does the job of postal workers, but quicker and more efficiently.CWU's Dave Ward wrote:The current cuts, bullying managers and ever-increasing workloads on a shrinking workforce cannot continue.
The CWU argues that in some centres, jobs have been cut but the technology has not been brought in, which has resulted in workers being over-stretched.
Mail services could be badly affected
"There are serious and growing problems in the postal sector which urgently need resolving," said Dave Ward, deputy general secretary of the CWU.
"We have renewed our offer of a three month no-strike deal to Royal Mail in return for meaningful talks over modernisation.
"The current cuts, bullying managers and ever-increasing workloads on a shrinking workforce cannot continue."
The Royal Mail has criticised the action, saying that it would "hurt both businesses and individuals".
It says 90% of its workforce will not be taking part in the strike and they will do all they can to ensure services are not badly affected.