Royal Mail workers taking part in 19 days of industrial action have said they will "do our best to avoid Christmas disruption".
Strikes were announced by members of the Communications Workers Union (CWU) on Tuesday, September 28.
Starting on Thursday, October 13, the walkout has been called for 24 hours from 4am on various days, including Black Friday week and Cyber Monday.
"We have not announced the strike action lightly," said Paul Moffat, regional secretary of the CWU's eastern region.
"But the fact is our members felt we needed to ramp this up.
"Having one, two or three-day strikes have not been taken seriously so that is why have had to push it this far."
The announcement has caused widespread concerns of disruption in the run-up to Christmas.
Areas of the company taking part in the walkout include those dealing with deliveries, distribution, processing and admin.
"We understand this will be disruptive and there will be some pain for long-term gain," said Mr Moffat.
"We are ding our best to avoid Christmas disruption and the heavy traffic of mail during these coming months.
"But we need to force the hand of chiefs to sit around the table and have some meaningful talks."
The strikes have been called over a dispute regarding pay and terms and conditions.
Royal Mail announced it would give members a non-conditional 2% pay increase backdated to April 1, alongside an additional 3.5% subject to further talks and agreements.
However, union leaders say any chance of potential negotiations have been rejected by the company.
Mr Moffat said: "Our members don't want to lose money and they don't want to go on strike.
"But they want a secure future and they want a secure job.
"The board is hellbent on destroying the company and turning it into a place of hire and fire and zero-hour contracts.
"We need to say enough is enough and get them back round the table."
CWU officials also claim any future backlog will be difficult to clear quickly because Royal Mail have "refused to offer overtime".
Speaking on the disruption, Mr Moffat said workers hope to find a resolution to avoid the extended length of strikes.
"A the end of the day, this issue has been caused by the board.
"We need the public to support us because we are supporting them in the long-term.
"CEOs pay themselves inflated bonuses that could have paid our members 20-fold.
"This is absolutely the right time for strikes. The difference between the big bosses and workers is huge."
A Royal Mail spokesman said: “On September 22, Royal Mail invited CWU to enter into talks through Acas to find a resolution to our dispute on change and pay.
“Rather than responding to our offer of Acas talks, the CWU announced further damaging industrial action, once again taking the path of prolonging disruption over resolution.
“Royal Mail is losing £1 million a day and must change faster in response to changing customer demands. We operate in a competitive market, and our customers have choices.
“Further strikes and resistance to transformation by CWU will only make our financial position worse, and threatens the long-term job security of our postmen and women.
“The CWU has a responsibility to recognise the reality of the situation Royal Mail faces as a business and engage urgently on the changes required.
“We apologise to our customers for the inconvenience the CWU’s continued strike action will cause. We are doing all we can to minimise any delays and keep people, businesses and the country connected.”
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