Some Inland Revenue workers were threatened with physical violence - and even received death threats - from people frustrated by non-payment of tax credit.
Long hours were voluntary say Revenue
According to a highly critical report issued by the PCS, the civil service union, tight deadlines for the introduction of the credits were not backed-up with appropriate resources.
Overall, the PCS report paints a picture of Inland Revenue staff weighed down with long hours and poor working conditions unable to cope with the introduction of the new credits.
The report will be a further body blow to the Inland Revenue and its political masters following the botched introduction of the tax credits which left many vulnerable families who should have received payments facing delay.
Height of problems
The new tax credits - the Child Tax Credit and Working Tax Credit - were introduced in April.
They have replaced three existing tax credits which were available to parents with children and disabled people.
BBC News Online received a large number of emails from readers who had become increasingly frustrated by non-payment of the credit.
At the height of the problems, hundreds of Inland Revenue workers took part in a "spontaneous" walkout because of the extreme pressure they were under.
At the time two million calls a day were attempted to the Inland Revenue helpline.
The National Audit Office is investigating the introduction of the tax credits. The PCS report is the union submission.
The report says the initial impact on PCS members employed to deal with the flood of calls from confused tax credit claimants was "to say the least a testing experience...they bore the brunt of customer frustrations and saw their workload more than double".
In addition, the PCS concludes the Inland Revenue was swamped with many employees having to work long hours and weekends.
Frequently PCS members were switched from regular duties to deal with tax credit queries.
According to the PCS report, Inland Revenue workers in offices throughout the country reported overcrowding, health and safety violations, assault and even death threats received from some claimants.
In some Inland Revenue offices emergency measures had to be introduced to cope with the flow of tax credit claimants.
In several places, hundreds of claimants queuing outside office were brought into temporary waiting rooms 20-30 at a time.
However, it often took an hour or more before people in the waiting room were seen by Inland Revenue advisers.
One anonymous Inland Revenue office worker wrote to the PCS with their concerns:
"I can foresee a major disaster happening if something is not done soon, had there been a fire there was not enough space for people sitting in the waiting area to be able to exit quickly.
"They would have tripped over themselves of somebody else, or a buggy or something."
In response, an Inland Revenue spokesperson told BBC News Online that such emergency measures were short lived and the system was now on an even keel.
"For a short period of time a small number of people did not receive the money they were due as fast as we would have liked.
"However, 90% of people were paid on time and as for staff working long hours it was entirely voluntary," he said.