More than 25,000 postal workers are to launch a series of strikes over the next few days in a worsening row over pay and jobs, threatening huge disruption to mail deliveries in the News Shopper area.
The Communication Workers Union (CWU) said the walkouts will hit postal services across Britain, including London, the West Country, East Anglia, the Midlands and Scotland.
The strikes will be the biggest outbreak of action since a national stoppage in 2007, with more disruption threatened in the coming months.
The union's executive has decided to hold a national ballot of its 160,000 postal members next month, which could lead to nationwide strikes in the autumn.
Strikes have hit several areas of the country, notably London and parts of Scotland, in recent weeks in the dispute over pay, jobs and services. The action is now spreading to other parts of the country, and for the first time will involve drivers of Royal Mail's huge lorries.
Drivers based in London, Northampton, Birmingham, Coventry, and Essex will be among workers striking for 24 hours on Friday, while postal staff in Somerset, Bristol and Edinburgh will take action on Saturday.
The strike will spread to Ipswich and Suffolk on Monday, with further areas set to be hit later next week.
The union said it had been forced to strike because cuts were being made to pay, jobs and services without agreement.
Deputy general secretary Dave Ward said the "panic driven" cuts were hitting postal services and accused the Royal Mail of imposing changes.
The Royal Mail has accused the union of reneging on a deal which ended the 2007 strike and was aimed at modernising the service.
A Royal Mail spokesman said: "Having met the union more than 50 times in recent months, it is increasingly clear that the CWU refuses to believe that mail volumes are declining despite the clear evidence of this.
"We condemn the CWU for striking over much-needed modernisation and change which has already been successfully implemented by our people in the majority of offices around the UK and is working well."
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