An Post has warned customers that it is not accepting parcels for the forseeable future because of the escalating dispute at parcel-delivery subsidiary SDS.
A company spokeswoman said this afternoon only parcels were affected and that people should post letters as normal.
Some 64 workers have been suspended at SDS in a row over the closure of the company.
Following talks at the Labour Relations Commission yesterday, the Communications Workers' Union (CWU) insisted that all suspended staff should be reinstated immediately to allow talks on arrangements for closing SDS to be completed.
The company began suspending workers after they refused to co-operate with management moves to proceed with the reintegration of SDS's business into the main An Post group.
The dispute was prompted by a refusal by SDS staff to load parcels which had arrived from the United States on to a lorry for transportation to Portlaoise, where they were to be processed under the company's new arrangements.
A company spokesman denied a claim by the union that a number of Dublin-based SDS workers had been ordered this week to relocate to Portlaoise.
The CWU say it is not the their intention to escalate the dispute but if the company insists on maintaining its position then other parts of the service would unavoidably be affected.
An Post is adamant that the decision to close SDS was taken after a "thorough and detailed review" of its business.