Postal services in Belfast were halted today after 350 staff walked out at the city's main delivery centre.
They left when a manager under investigation for an alleged assault returned to duty.
Deliveries to north, west and south Belfast were disrupted by the action which management described as "illegal".
Area manager Mr Michael Kennedy said: "This will be a real blow to customers who have every right to expect a consistent and reliable service without unforeseen and unjustifiable disruption."
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The assault allegation was found to have no substance, he added. Mr Peter Hamill of the Communication Workers Union said the strike action could continue for the foreseeable future.
PA