Talks to avert a postal strike in Preston have broken down.
The Communication Workers Union (CWU) said its members at the central delivery office on Christian Road will walk out at midnight for 24 hours.
The dispute follows the introduction of a new delivery system and unrest among workers regarding the way in which extra payments are allocated.
Union officials say the terms they were offered by management were not enough to prevent the industrial action.
Royal Mail says priority service items will still be delivered and it will do its best to deliver mail to businesses, but there will be no ordinary mail deliveries to homes in the area on Wednesday.
Collections are expected to take place as normal.