Members of the Irish Postmasters' Union are refusing to take on any new work for An Post due to a dispute over payments.
The union says it has taken the decision following a failure to reach agreement on pay increases for transacting existing business.
The dispute will affect the implementation and operation of a new joint banking venture between An Post and the Belgian firm Fortis.
IPU general secretary John Kane says the decision to refuse new work results from a litany of difficulties faced by postmasters going back a number of years.
He says IPU members are "at the end of their wits" from watching the postal network fall asunder due to lack of financial support from both An Post and the Department of Communications.