Engineers are trying to fix a massive computer failure at the Department for Work and Pensions (DWP) that has caused chaos in the benefits system. More than three-quarters of its staff could not access payment processing systems on Monday after an update to desk-top computers went wrong.
The DWP says the problems are "blown out of proportion" but added there may be delays to new and altered claims.
It said 95% of the problems are fixed and existing claims are unaffected.
About 80% of the DWP's network crashed on Monday and technical experts from Microsoft and the computer firm EDS have been working around the clock to find the fault.
Meanwhile staff were forced to communicate by fax because the e-mail system broke down.
The Conservatives are demanding to know the size of the backlog and how long it will take to sort out.
Tory pensions spokesman David Willetts said the problems were "more serious" than the government was making out.
He told BBC Radio 4's Today programme: "They have been very reluctant to tell us what happened.
"They said the problems were intermittent but in fact they were more serious than that."
A DWP spokesman denied there was any backlog at all, saying "disruption to customers would be minimal".
Alexis Cleveland, DWP's business community manager, said the department had business continuity plans to deal with the crash.
She explained: "20% of the systems were working, staff were able to access those systems and we were able to prioritise our work, minimising the impact on our customers."
Mark Serwotka of the Public and Commercial Services Union labelled the crash as possibly the biggest in public sector history.
He accused the government of "gambling with front line services" by relying too heavily on computer systems at the expense of people's jobs.
In a written parliamentary statement, the DWP said that in 2003/2004 it spent £412.5m on external management and technical support, including consultants, advisers, accounts and lawyers.
The breakdown was £306.7m on management and IT consultancy, £51.5m on staff substitutions and contractors and £54.3m on professional services.
A DWP spokesman said the increased use of outside consultants and advisers was required to implement "major modernisation programmes as planned".
"In particular modernising our legacy IT infrastructure, integrating business processes inherited from our predecessor organisations and modernising service delivery, including the introduction of Pension Credit and the DWP element of the new Child Tax Credit."
The DWP's modernisation programme was one of the largest of its kind in Europe, he added.
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