More than two million letters and parcels were piling up in sorting offices in the North-east because of a week-long stoppage by almost 1,000 workers at Newcastle upon Tyne.
Talks aimed at settling the dispute broke down yesterday. The strike is over the introduction of a new shift pattern, which the Post Office said would mean mail being delivered more quickly in the region.
The Communication Workers' Union rejected the new arrangements in a ballot and launched the strike, which is disrupting deliveries and hitting local firms.
A Post Office spokesman described the industrial action as "totally unnecessary".
He added: "If this strike continues over Easter, it will only be because the union has chosen to ignore the proposals we have made to end it.
"There is no mileage in the union continuing to hold our customers to ransom and inflicting disruption on local firms and families.
"Customers can be assured that we are doing all we can to get this dispute ended."
The spokesman, however, claimed there were enough volunteers to introduce the new shift system.