Royal Mail workers are holding the first of 19 strikes in a long-running dispute over pay and conditions.
The Communication Workers Union (CWU) said 115,000 members across the UK would walk out in a 24-hour strike on Thursday, starting at 04:00 BST.
General Secretary Dave Ward said workers faced the “biggest ever assault” on jobs, terms and conditions “in the history of Royal Mail”.
Royal Mail said further strikes would “weaken” its financial position.
The planned 19 days of industrial action include Black Friday week and Cyber Monday, as well as 13, 20, and 25 October, and 28 November.
Letters will not be delivered and some parcels will be delayed, the Royal Mail has warned.
This is the sixth strike for postal workers, and comes after a summer of unrest which saw rail workers and criminal barristers walk out amid disputes with their employers.
The CWU has accused Royal Mail of planning structural change. This would effectively see employees in secure, well-paid jobs turned into a “casualised, financially-precarious workforce overnight”, said the union.
It said plans by the postal service include cutting workers’ sick pay, delaying arrival of post by three hours and inferior terms for new employees.
General Secretary Mr Ward said the changes could lead to the “destruction of the special relationship that postal workers and the public have in every community in the UK”.
He described the plans as an “asset-stripping business plan” that will lead to the break-up of the company.
Royal Mail said it would do what it could to keep services running, but the strike was likely to cause disruption.
It also said it would prioritise the delivery of Covid-19 test kits and medical prescriptions.
A Royal Mail spokesperson said: “Royal Mail is losing £1m a day and must change faster in response to changing customer demands.”
Industrial action will threaten the job security of postal workers, said Royal Mail, calling on the leaders of CWU to cancel the walk-out and accept invitations for talks.
Royal Mail apologised for any delays to customers, adding: “We are doing all we can to minimise any delays and keep people, businesses and the country connected.”
