There is “no chance” of clearing that backlog before Christmas, according to Communications Workers Union representative Charlie Kelly.
He is among those postal staff who have tested positive.
No mail is expected to be delivered over the next few days.
More than half of the 124 staff at Royal Mail’s Great James Street sorting office are off after testing positive for coronavirus or having been in close contact with a colleague with the virus.
Mr Kelly said priority was being given to parcels, which means important letters such as hospital appointment correspondence and credit card bills are not reaching people.
“There are around 60 staff off in relation to Covid – either they have tested positive or they have been told to self-isolate – and there are another four off sick for other reasons,” he added.
“If 50% of the workforce is not available, then only 50% of the work can be done and that will create a serious backlog.
“It is as simple as that.
“No letters have been delivered throughout Derry over the past six or seven days while we try to get packages delivered.
“Realistically, there’s no chance of us getting caught up with the backlog between now and Christmas.
“It’s a nightmare.”
Mr Kelly said the amount of mail passing through the Derry sorting office had increased by around 40% since the beginning of lockdown in March.
He said Royal Mail needed to consider an alternative site that can safely accommodate staff adhering to social distancing rules.
A Royal Mail spokesman said: “We continue to deliver a limited volume of letters and will look to ramp this up over the next few days.
“Despite our best endeavours, customers in the Derry area may experience a reduction in service because of Coronavirus-related absences.”Tags: