The long-running postal dispute escalated last night after two fresh 48-hour strikes were called for next month and the Royal Mail pledged to press ahead with changes to services, including the scrapping of Sunday collections.
The Communication Workers Union said up to 130,000 of its members will walk out on October 5 and again on October 8, effectively crippling deliveries for five days, following the failure to reach a deal.
A rolling programme of strikes will start on October 15 and will continue every week until the row is resolved.
The threatened disruption to deliveries will be the worst since the dispute began earlier this year and will cost firms millions of pounds in lost business.
The union announced the new strikes after weeks of talks failed to break a deadlocked dispute over pay and jobs.
Dave Ward, the union's deputy general secretary, said: "Strikes are a proportionate response to an employer that is completely out of control. Rather than running the business, Royal Mail's actions demonstrate they are intent on destroying it."
Postal workers have staged four national strikes and other forms of industrial action since the summer after rejecting a 2.5 per cent pay offer and the Royal Mail's modernisation plans, which the union claimed would cost 40,000 jobs.
The union claimed Royal Mail was pressing ahead with changes to pensions, including increased employee contributions and later retirement age, and later starts to shifts.
The Royal Mail condemned the new strikes and said it would now start to implement the changes it insists are needed to modernise the business.
Sunday collections, which accounted for one per cent of all mail collections, will stop from the end of October, while early shifts will start an hour later from October 8.