A fresh wave of national strikes by postal workers will be held by the end of the month unless a bitter row over pay and conditions is resolved, union leaders announced today.

Talks aimed at resolving the dispute ended at the weekend without agreement and union leaders warned today that 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 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.

Royal Mail 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 later retirement age.

The two sides have also not agreed changes to conditions such as the start of early shifts.

A union spokesman said there had been "significant progress" in some areas including pay, and said the offer amounted to a "substantial" increase over the original 2.5%.

The union’s postal executive has agreed to continue talks with the Royal Mail but also warned that if no agreement is reached soon, strikes will be held before the end of the month.

The union will have to give seven days’ notice of any fresh industrial action.

The spokesman said the union had offered to continue the "period of calm" following its decision in the summer to suspend its campaign of industrial action to allow talks to continue.

"They have refused to extend the period of calm and they are indicating that they are going to start implementing changes from next Monday."

The union said it believed Royal Mail was about to scrap early starts, which would cost postal workers up to #25 a week and would lead to later deliveries.

A Royal Mail spokesman said: "The ’period of calm’ was an agreed period - yet we have told the union both yesterday and today that we are very happy to keep talking.

"There is no decision on the timing of operational changes. Postmen and women will not lose money.

"The changes we need to make are a direct result of EU transport legislation which restricts the speed of trucks and which means mail will get to delivery offices later - but we’ve offered that any postmen and women affected will keep their early start allowance in full as long as they continue in their current job even though they will be starting later.

"Our clear aim in consulting on pensions is to protect our existing people’s pensions as far as possible and not increase their contributions - while what the union wants is to keep the scheme open to new recruits, even though they know it will disadvantage our postmen and postwomen who already work for the company."