Stoppages are planned at postal centres across Britain
Thousands of postal workers are staging the first of what is expected to be a series of strikes amid a worsening dispute over jobs and modernisation.
The Communication Workers Union said a number of 24-hour stoppages would take place between Friday and Tuesday, after earlier strikes in London and Scotland.
The Midlands, East Anglia, the West Country, London and Edinburgh are among areas expected to be worst affected.
Royal Mail has condemned the action and says 90% of its staff will be working.
The strikes, involving more than 25,000 workers, will be the most widespread walkouts since national action in 2007.
Stoppages have affected several areas of Britain already in recent weeks, notably London and parts of Scotland, but the action is now spreading.
The latest round of walkouts will involve Royal Mail's lorry drivers for the first time, meaning that cross-country services are likely to be disrupted.
Friday Birmingham vehicle operation centre; Burslem; Coventry Parcelforce Hub; Edinburgh mail centre; Essex regional distribution centre; Huntingdon; London; Northampton national distribution centre; Peterborough delivery office
Saturday Somerset; Bristol delivery offices; Dalkeith; Edinburgh; Peterborough and Skegness
Monday Ipswich mail centre; King's Lynn; Stanton; Bury St Edmunds and Thetford in Norfolk
On Friday staff based in Birmingham, Coventry, Northampton, Peterborough, Huntingdon, Essex, London and Edinburgh will stage a 24-hour strike, and postal workers in Bristol, Peterborough, Somerset, Skegness, Dalkeith and Edinburgh will take action on Saturday.
Stoppages will spread to Suffolk and Norfolk on Monday, and further areas, including Stoke-on-Trent, are due to see disruption on Tuesday.
The Communication Workers Union (CWU) and Royal Mail accuse each other of reneging on a 2007 deal which was aimed at modernising the service.
CWU deputy general secretary Dave Ward told the BBC that Royal Mail management was not doing enough to convince workers of the case for change.
He said: "You've got to set out a vision of the future that people buy into.
"All our members see is endless cuts in services, endless cuts in jobs, attacks on their pensions, attacks on their pay and their terms and conditions.
"What we're saying is, nobody in their right mind would actually support that type of approach because they don't see any light at the end of the tunnel."
The union is now planning to hold a national strike ballot next month, which could lead to further disruption in the autumn.
A Royal Mail spokesman said it was "increasingly clear" the CWU was refusing to accept the reality of declining mail volumes.
The service, which employs around 180,000 staff, is suffering as a result of increased competition from other operators, the impact of the growth of e-mail and has a large pension deficit.
Royal Mail director of operations Paul Tollhurst told the BBC: "We are trying to deal with a market which has seen over a 10% fall in volumes in the last year.
"What we're trying to do is finish deploying the agreement we reached with the union in 2007, while we talk to them about all of the changes we need to make in the future.
"I'm saying that they have written to their branches to say 'do not co-operate with change', and that change specifically is encompassed in an agreement they signed in October 2007."
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