Royal Mail workers are holding the archetypal of 19 strikes successful a long-running quality implicit wage and conditions.
The Communication Workers Union (CWU) said 115,000 members crossed the UK would locomotion retired successful a 24-hour onslaught connected Thursday, starting astatine 04:00 BST.
General Secretary Dave Ward said workers faced the "biggest ever assault" connected jobs, presumption and conditions "in the past of Royal Mail".
Royal Mail said further strikes would "weaken" its fiscal position.
The planned 19 days of concern enactment see Black Friday week and Cyber Monday, arsenic good arsenic 13, 20, and 25 October, and 28 November.
Letters volition not beryllium delivered and immoderate parcels volition beryllium delayed, the Royal Mail has warned.
This is the sixth onslaught for postal workers, and comes aft a summertime of unrest which saw obstruction workers and transgression barristers locomotion retired amid disputes with their employers.
The CWU has accused Royal Mail of readying structural change. This would efficaciously spot employees successful secure, well-paid jobs turned into a "casualised, financially-precarious workforce overnight", said the union.
It said plans by the postal work see cutting workers' sick pay, delaying accomplishment of station by 3 hours and inferior presumption for caller employees.
General Secretary Mr Ward said the changes could pb to the "destruction of the peculiar narration that postal workers and the nationalist person successful each assemblage successful the UK".
He described the plans arsenic an "asset-stripping concern plan" that volition pb to the break-up of the company.
Royal Mail said it would bash what it could to support services running, but the onslaught was apt to origin disruption.
No letters volition beryllium delivered during onslaught days, said Royal Mail, but arsenic galore peculiar transportation and Tracked24 parcels arsenic imaginable would beryllium delivered.
It besides said it would prioritise the transportation of Covid-19 trial kits and aesculapian prescriptions.
A Royal Mail spokesperson said: "Royal Mail is losing £1m a time and indispensable alteration faster successful effect to changing lawsuit demands."
Industrial enactment volition endanger the occupation information of postal workers, said Royal Mail, calling connected the leaders of CWU to cancel the walk-out and judge invitations for talks.
Royal Mail apologised for immoderate delays to customers, adding: "We are doing each we tin to minimise immoderate delays and support people, businesses and the state connected."