West Sussex county council has blamed "teething issues" for its failure to collect nearly £500,000 of social care payments from residents.
The local authority said delays in a new IT system for adults' services, designed to integrate care planning, purchasing and charging, led to the error. It said it was working to resolve the problem, which meant more than 500 residents underpaid for care packages and equipment.
Arrangements have been put in place to collect the underpayments from those affected. Customers have been asked to begin payment of the arrears in February, with two temporary finance staff employed until March 2012 to work through the data and manually process the backlog at a cost of £21,000.
West Sussex said that it will be particularly sensitive about approaching a family if a resident has died or claiming money back where there are genuine hardship cases.
A spokesman for West Sussex county council said: "The county council has been very open about the difficulties we have experienced with the new IT system. There have been delays in billing some customers for services received, for which we apologise. This has meant we now need to catch up on invoicing those affected for care services they receive.
"We have already contacted all affected customers to let them know about this problem. We would like to emphasise that it is money they pay for their normal services which has not been collected over the past few months."
He said that suggestions that the council was attempting to collect a tax on benefit payments on behalf of central government were "completely untrue".
"Sadly, this just creates confusion for vulnerable people," he said.
Despite the debacle, the council said it satisfied with the service was being provided and that the automated technology would being operating again once the technical issues had been resolved.
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