More than 12,000 postal workers are on strike as of Friday in a row over jobs, pay and services.
The 24-hour strike will affect cities ranging from Edinburgh to Plymouth. The union has accused Royal Mail of cutting the pay of employees and reducing services.
Dave Ward, the union's deputy general secretary, said: "There are serious and growing problems in the postal sector which urgently need resolving.
"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.
"Pressure and stress is at breaking point for postal workers."
Workers based in London, Edinburgh, Bristol, Darlington, Stoke-on-Trent, Plymouth, Norfolk and Essex will be taking part in the strike. Protest letters will be delivered to the Royal Mail's chief executive, Adam Crosier, and Business Secretary Lord Mandelson.
The union said it was receiving an "ever-growing" number of requests for industrial action from postal workers across the country. It said 400 requests for ballots on strike action have already been made and Mr Ward warned that without progress the dispute would turn into a national strike.
Royal Mail said the CWU was objecting to changes its leaders had agreed to in 2007.
Controversial plans to part-privatise Royal Mail were put on hold by the government in June. The bill was due to go before Parliament before the summer break, but Lord Mandelson said it would not happen until "later".
I got post today. For a
I got post today. For a second I thought maybe they were doing what they've done before and doing a staggered strike, then I realised it must have been a scab.
I don't think we had any
I don't think we had any delivery or pick up from work today.