It used data from the 2006 Competitive Market Review by industry watchdog Postcomm to come up with the figure, based on how much franked mail is sent each day by smaller enterprises.

A BCC spokeswoman said:

"Most smaller sized businesses have no option but to use Royal Mail and collectively spend millions of pounds each day on their post.

This series of strikes is doing nothing but cause delay and frustration for those who rely on the postal service and the sooner it is resolved the better."

The most recent strike, which will effect different parts of Royal Mail over a fortnight, began on Wednesday night.