A series of 24-hour strikes by postal workers is expected to continue in an ongoing dispute over pay, jobs and services.
Members of the Communication Workers Union (CWU) in Stoke-on-Trent are due to walk out, with industrial action planned in London on Wednesday, spreading to the South West on Thursday.
This follows industrial action in East Anglia on Monday and other locations last Friday and over the weekend.
The union has announced plans to ballot all its postal-worker members for a national strike in September, threatening the worst disruption to mail deliveries for two years.
The union's deputy general secretary Dave Ward said: "Royal Mail management is trying to crush the British postal service. They have been criticised by Government for failing to tackle industrial relations problems, yet they still refuse to address the concerns of postal workers.
"Postal workers and customers are experiencing cuts to services, cuts to hours, cuts to jobs and threats over future jobs and services. This is just downsizing, there's nothing modern about it."
CWU spokeswoman Sian Jones said that Royal Mail had refused a three-month "no strike" deal in return for negotiations over changes to the business.
She added that a national strike would be a "last resort" which members are hoping to avoid.
Royal Mail accused the union of breaking the agreement that ended the 2007 strike and of refusing to co-operate with modernisation.
A spokesman said: "Royal Mail has a clear vision of creating a modernised and market-leading business in which we are investing around £2 billion. We again call on the CWU to help us secure a strong future for the business, its people and its customers rather than repeatedly acting to undermine it."
