A fresh wave of postal strikes is set to be held in an escalation of the bitter Royal Mail dispute.

As thousands of postal workers mounted picket lines in the first of two 24-hour walk-outs, further industrial action is expected to be announced.

The next wave of strikes is likely to be held for three days at the end of next week, involving different groups of workers. The move would cause further disruption to mail deliveries, which are already facing big delays because of this week's stoppages.

Prime Minister Gordon Brown urged Royal Mail management and postal workers to get "round the table" to solve the dispute, saying the strike was "self-defeating".