Royal Mail managers were today accused of being "industrial anarchists" as unofficial strikes by postal workers escalated, causing huge disruption to services.
The Communication Workers Union said up to 20,000 workers in London had joined a series of wildcat strikes at mail centres and offices across the capital.
Postwatch warned of a winter of discontent in the postal service unless the union and Royal Mail regained control of industrial relations.
CWU deputy general secretary Dave Ward said the disputes resulted from local managers "attacking, humiliating and belittling" union members who took official strike action two weeks ago in a row over London Weighting allowances.
"When our members returned to work after a day's legal and democratic strike action, managers made every effort to humiliate and provoke them.
"The list of petty penalties they tried to impose guaranteed a reaction from postal workers.
"At various places they refused overtime, changed duties, insisted on working Saturday as a normal day, victimised union representatives and generally tried to change, without agreement, all kinds of our members' terms and conditions.
"The union is not backing unofficial strike action – but neither can we deny that this amount of management provocation almost guaranteed a reaction from our members."
Postwatch said services in the capital had been "severely disrupted" for the past 11 days. Customers did not know when their mail would be delivered.
More than 20 delivery offices were hit and the action had spread to mail centres at Nine Elms and Whitechapel, which processed the bulk of London's mail, the postal watchdog said.
It said it had been inundated with calls from "anxious and disgruntled" customers.
Peter Carr, chairman of Postwatch, described postal services in London at the moment as "appalling".
He added: "It is too much of a coincidence for more than 20 unofficial strikes in London to take place in 10 days and not be connected to the CWU's campaign for improved London Weighting allowances.
"The union's repudiation of this unofficial action has a hollow ring. This could herald the return to the persistent industrial unrest for which Royal Mail earned an unwelcome reputation.
"The management and unions must regain control and restore the capital's postal services – otherwise we are in for a winter of postal discontent.
"Customers need to know when services will be back to normal."
Mr Carr said it was very disappointing that improvements in industrial relations over the past few years appeared to be disappearing in a wave of unofficial action that was "seriously undermining" services throughout London".
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