Staff and management are still at odds on the issue of modernisation

Workers in mail centres and at airports will strike for 24 hours from 1900 BST, followed by action from other groups of employees including delivery staff.

The Communication Workers Union has already staged two 24-hour strikes over a 2.5% pay deal and modernisation plans which it says threaten 40,000 jobs.

Royal Mail said the union was ignoring the "harsh reality" of an open market.

The union has said that a modernisation programme will soon be imposed on part of the workforce and will mean "significant" changes including later delivery start times.

Deputy general secretary Dave Ward said: "The imposition represents an unnecessary attack on postal workers' jobs, pay and conditions."

He also said that Royal Mail should "engage in meaningful negotiations".

Stoppages by postal workers will now be held across the period from 9 to 17 August.

Mail bosses have denied the union assertion that changes would be made from next week under the modernisation plans.

And a Royal Mail spokesman added: "In taking further strike action the CWU is again failing to grasp or understand the harsh realities of an open postal market and the intensifying competition from other forms of communication, including broadband internet."