Some postal workers at Mallusk blocked the path of mail vans

It also said mail posted in Belfast will not be processed or delivered.

The announcement comes as the week-long postal dispute in north, south and west Belfast continues. Deliveries in the east of the city have continued.

The national leadership of the Communication Workers Union will hold a mass meeting on Tuesday when strikers will be advised to go back to work.

The CWU said it "repudiated last Tuesday's unballoted strike action" and union official Ray Ellis said he would address the meeting.

The strike over bullying allegations began at the Tomb Street depot but spread to the main sorting office at Mallusk on Friday.

Mr Ellis said the strikers should return to work, adding that feelings "are running extremely high".

"The people I have spoken to are clearly very angry and very concerned about what they perceive are the underlying problems that have triggered this walkout," he said.

"I don't think it's been helpful that Royal Mail have personally attacked the local represenatives through the media and by threatening them with legal action."

The Royal Mail said a majority of workers at the Mallusk depot turned up on Sunday night and again on Monday.

We are in a position where our door is still open, we want to talk to people about getting a return to work David Peden Royal Mail

Royal Mail watchdog Postwatch warned on Monday that postal services across Northern Ireland would be affected if the dispute was not settled.

Meanwhile, about 30 striking postal workers staged a protest outside Hillsborough Castle on Monday where Northern Ireland's political parties were meeting with the British and Irish governments.

The workers have asked the parties to show support for their strike action.

Disciplinary procedures

The unofficial action was started by more than 200 postal workers at the city's Tomb Street depot.

It was blamed on difficulties between staff and management over disciplinary procedures and other issues in the north Belfast section.

Royal Mail has said workers need to come off the picket line before they will enter into negotiations with them.

However, staff involved in the dispute have said they have no intention of returning to work until the original dispute in Belfast has been resolved.