Postal unions have announced a further wave of strike action in an escalation of their dispute with Royal Mail over pay and modernisation.
Unions say they have overwhelming support for the stoppages
The current period of staggered stoppages was due to end on 7 August.
But the Communication Workers' Union (CWU) said stoppages will now also be held from 9 to 17 August.
The union has called for "serious negotiations", but Royal Mail has said its pay demands threaten efforts to make the business more efficient.
"Despite Royal Mail's stated position and with the prospect of imposed change, the Union has made a fresh offer for a period of calm," said CWU deputy general secretary Dave Ward.
"We have made yet another offer... to call off the next planned strikes. All Royal Mail have to do is take a step back and engage in meaningful negotiations."
Exploratory talks facilitated by the mediation group Acas have taken place this week between the CWU and Royal Mail.
Royal Mail said it was at a loss to understand why the union had announced more strike dates while conciliation talks were taking place, and urged the CWU to call off the action.
"Royal Mail...deplores the move by the union, in spite of these talks, to attempt to cause further disruption to customers' mail," a spokesman said.
"As the CWU is responsible for calling the strikes, they are clearly free to stop this damaging action at any time."
Workers are in the middle of a two-week period of staggered stoppages designed to maximise disruption to the business.
They have already staged two 24-hour strikes in protest at what they say is an inadequate 2.5% pay offer and modernisation plans which threaten 40,000 jobs.
More than 98% of staff took part in Tuesday's stoppages at mail centres across the country, the union said.
Further stoppages at delivery and collection centres are due later this week.
The CWU said a backlog of 80 million items of mail had now built up in the system.
Royal Mail has said it has made contingency plans for further stoppages to ensure that services are not affected.
The company has called for a return to negotiations but has warned unions that fundamental changes are needed if it is going to survive in a fully competitive market.