A post box is seen in London October 29, 2009.
Talks between management and unions to try and break the deadlock are due to resume under the auspices of the TUC on Tuesday, the union said.
Two more 24-hour strikes, involving all 21,000 postal workers across Britain, are set for Friday and next Monday.
(Reporting by Stefano Ambrogi; Editing by Steve Addison)