The postal workers union has said it will announce further strikes after failing to reach an agreement with the Royal Mail during five days of talks.
The CWU fears Royal Mail's shake-up plan could cost 40,000 jobs
Industrial action had been scheduled for last week, but was suspended for last ditch negotiations at the weekend.
According to the Communication Workers' Union (CWU), while progress had been made in some areas, including pay, it was not enough to avert strike action.
CWU members are concerned about pay levels and modernisation plans.
The Royal Mail said it was "extremely disappointed" the CWU had continued "to ignore the stark challenges facing the company".
Since June, the CWU has staged a number of national walkouts in protest at Royal Mail's offer of a 2.5% pay increase and the company's aggressive modernisation plans.
The union claims the shake-up plans will put about 40,000 jobs at risk.
According to the CWU, the Royal Mail has now upped its pay offer to a two-year increase worth 6.7%, however the company is sticking to pension proposals rejected by the union - such as increased employee contributions and later retirement age.
But the Royal Mail countered that it had "made it clear" to union officials that their aim was to "protect (our) existing people's pensions as far as possible and not increase their contributions".
The CWU also said there has been no final agreement on a number of areas including job security, personnel procedures, automation, productivity and network changes.
Until now, Royal Mail has stood firm against union demands insisting they are unaffordable.
Royal Mail added it could no longer delay "the next steps" in modernising the business, if it is to battle increased competition in the sector after the opening up of the UK postal market and finance the upgrading of its equipment.
But the CWU says the move ends an agreed period of calm, during which Royal Mail had promised not to introduce changes to its operations, in exchange for a commitment from the union not to call further strikes.
Fearing that the Royal Mail is about to "implement unagreed operational changes", the CWU said it would announce further strikes which will take place before the end of September.