Postal staff in the North are considering strike action after three Catholic workers received death threats from loyalist paramilitaries.
The threats were made against two managers and a postman based at a sorting office in Belfast. The Royal Mail has said it is taking the threats seriously and is working with police to improve security for employees.
Last year, Catholic postman Mr Daniel McColgan was shot dead by the UDA as he arrived for work in Newtownabbey, on the outskirts of north Belfast.
Postal deliveries were suspended for two days following his murder and rallies were held across the North in protest.
The latest threats were made in the name of the Red Hand Defenders, a cover-name for the UDA.
A Royal Mail spokesman said: "These threats are now the subject of a police investigation. We are doing all we can. The safety of our staff is paramount and we are taking these threats very seriously."
A spokeswoman for the Police Service of Northern Ireland (PSNI) said she could not discuss the personal safety of individuals.
Mr Peter Bunting of the Irish Congress of Trade Unions described the threats as disgusting.
"Postal workers are vulnerable. They are out there on their own doing a job in very dark hours of early mornings and they are the easiest targets walking around the streets of Northern Ireland.
"Workers have the right to go about their work without fear from anyone or threats from anyone and I think this is a deplorable week in Northern Ireland for workers."
SDLP Assembly member, Mr Alban Maginness, condemned those who had issued the threats.
"I was contacted by one postal worker who had received a threat. He was very upset. These threats must be taken very seriously following the murder of Daniel McColgan. They must be withdrawn immediately.
"I understand the Communication Workers' Union is considering strike action. I would urge them to take a strong stand against sectarian threats."
Meanwhile, dissident republicans are believed to have been responsible for a bomb attack on a PSNI base in west Belfast.
A blast bomb was thrown at Woodbourne station on the Stewartstown Road at 11 p.m. on Thursday. It landed inside the perimeter fence but failed to explode. It was later made safe by the British army bomb disposal unit. Police described the device as "viable but crude".