About 400 postmen and women are leaving Royal Mail per week, according to the Communication Workers Union (CWU), who claims this number has gone up since the company introduced a new delivery system earlier this year.
The CWU says the firm had been losing about 330 postal staff a week from a workforce of 83,000 before it rolled out DM26, which reorganised routes to improve delivery performance.
The union claims the new system is not working in hundreds of offices, and BBC Your Voice has heard from staff warning of a "nightmare" Christmas ahead as the number of parcels increases.
Royal Mail says the reforms are part of its plan to deliver a more "efficient and financially sustainable" service.
Over the last year we've been in touch with more than 30 postal staff in all four nations of the UK, as they have switched over to DM26.