One million callers to a new automated phone line for benefit claims failed to get through in just six months earlier this year, according to reports today.
The CMS system, operated for Job Centre Plus by US computer giant EDS, allows clients to register their details before being called back days later by an official in order to go through their application in full.
But a BBC investigation found that an average of one-third of calls to the computerised lines between April and September went unanswered, with the proportion of unanswered calls rising to two-thirds at a call centre in Sheffield.
Many claimants spent hours trying to get through and there were then lengthy delays before they were called back and further waits before their claim was processed, the Radio 4 Today programme reported.
Unions said that claimants for benefits like income support and jobseeker's allowance were waiting as long as eight weeks for their first payments - many of them with no other financial resources to draw on.
Mark Serwotka, general secretary of the Public and Commercial Services Union (PCS), called on the Government to halt a round of 30,000 job cuts at the Department for Work and Pensions.
He said desperate claimants were venting their frustration on frontline workers, with a 66% increase in assaults on staff since the computerised system was introduced.
"We think this is an inevitable consequence of (Chancellor) Gordon Brown's efficiency drive," Mr Serwotka said on Today.
"He says computers can replace people. This is categorical proof that not only do the computers not work, but that when you force vulnerable people to go through call centres to access the benefits system, if there's not enough staff and computers don't work, one million people have tried to get through and can't."
Clients who turned up at Jobcentre Plus offices were being turned away because they had not contacted the phone line first, and the calls were not even free, so some of the country's poorest people were paying large amounts for hanging on the line, he added.
Mr Serwotka went on: "We believe the system of removing local access to the benefit system is fundamentally flawed. The Government has invested millions in this and we doubt that they will go back on it.
"Our view is that until all this works perfectly, all the staff cuts must stop. Fifteen thousand jobs have gone already, they are planning to cut another 15,000.
"We are calling on them to stop these reductions and ensure that while the system is not working, our members are not asked to work under intolerable pressure and the public are not asked to wait eight weeks to get on to the system."
Mr Serwotka predicted that the problems would get worse in January.
But a spokesman for the DWP said that, while problems had been experienced by claimants earlier this year, the situation was improving. Some 90 per cent of calls were getting through last week, according to the spokesman.Reuse content