Union leaders say huge backlogs of post have built up in recent weeks
Postal workers across the UK have gone on strike again as part of a long-running row over modernisation plans.
The latest 24-hour walkout is affecting deliveries in locations including Bridgend, London and Glasgow, and has added to a growing post backlog.
It comes ahead of a national ballot for strike action among members of the Communication Workers Union next week.
Royal Mail said the strikes were hurting customers and called on staff to return to work.
The CWU is accusing Royal Mail of forcing through changes to the postal service - including cutting pay and jobs - without proper consultation.
Other areas affected on Saturday include Birmingham, Carlisle, Coventry, Northampton, Warrington, Swindon, Bristol and Leeds.
Members of the CWU will begin voting on Wednesday on whether to support a national strike.
The result is expected on 30 September - during the Labour party conference.
If a country-wide walkout is approved it would be the biggest disruption to the postal service since 2007.
Royal Mail called the decision to hold the ballot "wholly irresponsible" given that talks between management and the union were still being held.
"We urge the CWU to abandon strikes and the threat of strikes, and focus on providing customers with the service they need and expect, rather than planning to hurt them with the threat of more strikes," operations director Paul Tolhurst said.
Strikes have taken place throughout the UK since June and more are planned for next week, the first at delivery offices in Cambridgeshire on Monday.
The CWU's deputy general secretary, Dave Ward, said: "Royal Mail's head-in-the-sand approach to the problems in the mail industry is now severely damaging services for customers, with backlogs bigger than in the national strike of 2007."
Earlier this week, the union claimed that "mountains of undelivered mail" were lying in delivery offices around the UK, including more than 20 million items in London.
But Royal Mail played down the scale of the problems and said in a statement on Saturday: "The vast majority of deliveries are taking place as usual today all over the country.
"And, with 90% of our people working normally, Royal Mail is doing all we can to minimise disruption to those customers in the areas which have experienced industrial action."