Royal Mail managers are being told to work over the weekend to clear a post backlog in south Warwickshire.
There are believed to be tens of thousands of items undelivered in the Stratford area following changes to working practices.
The Royal Mail has apologised for the disruption, blaming snow and new working arrangements.
Delays in the Stratford depot have caused disruption in Warwick, Rugby and Kenilworth.
Communication Workers Union spokesman Dave Hatton said: "Introducing people with vans or lightweight lorries is actually slower than people cycling or walking with a pouch.
"It's safer, and there are lots of other advantages to it, but it's slower. Royal Mail never appreceated that."
Last month the Royal Mail said it was "pulling out all the stops," and met Conservative MP for Warwickshire, Nadhim Zahawi, to discuss the delays.
It's not just south Warwickshire. It took seven days for a 1st Class item to reach Hertfordshire from Coventry and another seven days for a 2nd Class letter about the poor postal service to reach Coventry
