The main postal union, the Communication Workers' Union (CWU), has announced further details of its forthcoming UK-wide strike action.
The CWU fears Royal Mail's shake-up plan could cost 40,000 jobs
Its 130,000 Royal Mail members will walk-out for 48 hours between noon on Thursday, 4 October, and the same time on Saturday, 6 October.
A second 48-hour strike will take place from 0300 BST on Monday, 8 October, to the same time on Wednesday, 10 October.
The union is in dispute with the Royal Mail over pay and job cut fears.
At the centre of the dispute is the CWU's objection to the Royal Mail's 2.5% pay offer and modernisation plans.
The union claims the shake-up plans will put about 40,000 jobs at risk.
Royal Mail counters that without dramatic reforms it will not to able to survive in a liberalised mail market.
The strikes will go ahead failing a breakthrough in talks between the two sides, which have now continued for six weeks without agreement being reached.
A second postal union, the smaller Unite, has also threatened to ballot its members over strike action in a dispute over pensions.
Unite, which represents 12,000 Royal Mail managers, says the organisation wants to close its pension scheme to current members and raise the retirement age.
Royal Mail insists no decision has been made, and that no change will be made without full consultation.