Talks aimed at resolving the dispute ended at the weekend without agreement and union leaders warned today that the Royal Mail was about to implement changes to the conditions of postal workers.
The Communication Workers' Union said: "Despite being committed to further talks and being determined to reach an agreement, CWU will announce further strikes to be held prior to the end of September."
The union said that the Royal Mail had improved its original 2.5% pay offer, which was heavily rejected by postal workers and led to a series of national walk-outs over the summer, which crippled postal services.
The company is now offering a two-year deal worth 6.7%, according to the union.
But officials complained that the organisation was sticking to rejected proposals on pensions including increased employee contributions and an increased retirement age.
The two sides have also failed to agree changes to conditions such as the start of early shifts.
The CWU's announcement, made on the second day of the annual TUC congress in Brighton, raises the spectre of a winter of discontent for Gordon Brown.
Mr Brown warned unions in his keynote address to TUC delegates yesterday there would be "no loss of discipline" over public sector pay in the future.
His comments were met with warnings from Mark Serwotka, the general secretary of the Public and Commercial Services union, that his civil service members would take part in a ballot over pay unless the government held "meaningful negotiations" before the end of the month.
Later today, the TUC congress will debate a motion on public sector pay, calling for coordinated industrial action against "unfair public sector pay limits". A Royal Mail spokesman said that elements within the union were "determined to drive on with this dispute".
"We have now been negotiating with the CWU over pay and flexibility since March 2007," he said.
"Royal Mail can no longer delay the next steps in modernising the business to enable it to compete on an equal footing with other operators and will now begin to make the changes which have been discussed with our people and our trade unions for many months.
"It has become clear during this time that certain elements within the CWU are determined to drive on with this dispute irrespective of the damage it is causing to our people, our customers and the future of Royal Mail."
