Royal Mail said the union was resisting "much-needed" reforms
Over 25,000 postal workers will stage a series of strikes from Friday to Tuesday over pay and jobs, the Communication Workers Union has said.
Services across the UK, including in London, Scotland, the West Country, East Anglia and the Midlands will be affected, the union added.
London and Scotland have already witnessed stoppages in recent weeks.
Royal Mail said over 90% of staff would keep working and the "vast majority" of services would operate normally.
The strikes will be the most widespread walkouts since a national action in 2007.
On Friday drivers based in Birmingham, Coventry, Essex and London will stage a 24-hour strike, and postal workers in Bristol, Somerset and Edinburgh will be among those taking action on Saturday.
Stoppages will spread to Suffolk on Monday, and further areas are due to see disruption later in the week.
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 Edmonds and Thetford in Suffolk
Tuesday - Stoke-on-Trent
CWU deputy general secretary Dave Ward said "panic-driven" cuts were being made to jobs and services without agreement.
He added: "Postal workers are sick and tired of an incompetent management running their business into the ground.
"Workers are busier than ever and being treated badly."
But a Royal Mail spokesman said the union was failing to acknowledge "clear evidence" of declining mail volumes.
The spokesman said: "The CWU's real agenda is to block change and modernisation at Royal Mail and to absolutely oppose on the ground our goal of making Royal Mail a strong and innovative leader in the UK and international postal markets".
He added: "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."