The postal workers marched to the Belfast rally

Ray Ellis, a national Communication Workers Union (CWU) representative, told them their action was unofficial and said they should return to work.

However, beforehand, Mr Ellis told the media Royal Mail should also agree to a review of employee relations.

Post has been disrupted in north, south and west Belfast. Deliveries in the east of the city have continued.

Mail posted in Belfast is not being processed or delivered.

On Tuesday Mr Ellis said: "If they continue to refuse to talk, then I'm afraid our members are going to face more loss of money and customers are going to face more disruption.

"We are doing our best to get people back, Royal Mail need to respond."

The rally was held at Transport House, near the Tomb Street depot where the strike over bullying allegations began last Tuesday.

Action spreading

The action spread to the main sorting office at Mallusk on Friday, sparking fears the dispute could escalate throughout Northern Ireland.

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

Meanwhile, the BBC has obtained a letter sent to postal members of the GMB union by a union official.

It said "decent people are losing pay for very spurious reasons" and that the strike "has become deeply sinister".

The official who wrote the letter also stated: "This is the worst example of senseless action I have ever witnessed."

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.