Postal workers in Weston-super-Mare are preparing to strike after failing to reach a pay agreement with managers.
Postal workers were promised a pay rise for the extra workload
Workers claim they have not received a pay rise, promised when the second post was phased out.
But the Royal Mail says the increase is linked to productivity targets, which the Weston office has failed to reach.
Managers will meet with union leaders on Friday for crisis talks. But if no agreement is reached, staff will strike on Saturday and Monday.
It would affect deliveries to tens of thousands of homes and businesses in the BS22, 23 and 24 areas.
But the Royal Mail is poised to draft in managers from as far afield as Truro, Chester and Reading to keep them running.
Postal workers nationally were offered an extra £26.28 per week to compensate for the increased workload.
Martin Holloway, Delivery Sector Manager for the Bristol and Weston area, said the pay deal had to be funded by productivity.
"The vast number of offices across the country have reached this, but we are now in the final few yards in Weston. The last thing we need is strike action," he said.
Kevin Beazer, regional secretary for the Communication Workers Union (CWU) claims Royal Mail has not give a commitment in writing to award the pay rise .
"They are asking us a lot to give up our democratic right to ballot on a promise - a promise they have been making since June," he said.