Disabled people are facing "distress and financial difficulties" as a result of the slow processing of claims under a new Government benefit scheme, a spending watchdog's report has found.
The National Audit Office found that claimants for the new Personal Independence Payment, which will replace Disability Living Allowance, were waiting an average 107 days - and terminally ill patients 28 days - for a decision on their cases, rather than the predicted processing times of 74 days and 10 days respectively.
Within six months of the introduction of PIPs in some areas of the north of England in April 2013, a backlog of 92,000 cases had built up with private contractors Atos and Capita and the Department of Work and Pensions had made decisions in only 16% of the expected number of cases.
Poor operational performance in the early stages of Iain Duncan Smith's flagship programme has forced the Department to stagger the full national roll-out of PIPs and increased the risk that it will not deliver value for money in the long term, said NAO chief Amyas Morse.
Delays in assessments have cut by £140 million expected savings over the course of this Parliament, with the DWP now forecast to save £640 million a year by 2015, rather than its prediction of £780 million, said the NAO. However, the DWP still expects to achieve annual savings of £3 billion by 2018/19, with 3.6 million claims assessed by 2018.
Each new PIP claim - worth between £21 and £134 a week to disabled claimants - costs an average £182 to administer, compared to £49 under DLA, said the report.
"It is too early to conclude on the Personal Independence Payment programme's overall success and all major programmes run the risk of early operational problems," said Mr Morse. "However the Department did not allow enough time to test whether the assessment process could handle large numbers of claims.
"As a result of this poor early operational performance, claimants face long and uncertain delays and the Department has had to delay the wider roll-out of the programme. Because it may take some time to resolve the delays, the Department has increased the risk that the programme will not deliver value for money in the longer term."
The chairwoman of the House of Commons Public Accounts Committee, Margaret Hodge, said: " I was shocked to learn that, not only will Personal Independence Payment claims cost almost three and half times more to administer than Disability Living Allowance, they also take double the amount of time to process.
"The current backlog and delays in processing claims are simply unacceptable and will no doubt cause real distress for vulnerable claimants. Without the Department for Work and Pensions sharing details of how long claims should and do take, claimants are left facing uncertainty and potential financial difficulties whilst waiting for a decision.
"Once again we see the Department under-performing - and we have little faith that costs will not increase down the line as it tries to get things under control.
"The Department need to understand the causes of this backlog to develop a clear plan on how they are going to work with contractors to clear it, and ensure there are suitable processes in place to make sure this does not happen again."
Responding to Mrs Hodge, a DWP spokesman said: "PIP is a completely new benefit with a face-to-face assessment, something missing under DLA, and there will always be initial costs so this is not comparing like with like.
"The figures show good value for money for taxpayers in the short and long term, with expected savings of £3 billion annually by 2018/19.
He said the previous system was "b roken", adding: "Personal Independence Payment is a principled reform which will ensure support is focused on those with the greatest need. It includes a new face-to-face assessment and regular reviews, something missing under the old system.
"The NAO acknowledges this reform started on time and on budget, and we have reduced risk by rolling it out in phases. This has enabled us to adjust our plans as we learn from the initial phases, well before the roll-out to the majority of existing Disability Living Allowance claimants next year."
Steve Winyard, head of campaigns and policy at the Royal National Institute of Blind People, said: "RNIB is alarmed to note that 'early operational performance has been poor' and we're very concerned that DWP has only been able to process 16% of applicants, leaving 92,000 disabled people - including blind and partially sighted people - waiting for vital help.
"The process must be urgently improved."
Sarah Lambert, head of policy at the National Autistic Society (NAS), said: "This report shows that far too many people are experiencing unacceptable delays when trying to access PIP and that an unmanageable backlog of claimants awaiting assessment has developed.
"Disability benefits are a necessity, not a luxury, so such delays cause real financial difficulties for those who rely on them to get the support they need to live and work. It's deeply concerning that the DWP is unable to inform applicants how long they are likely to wait. This uncertainty will take an additional toll on people with autism, who often rely on routine and experience high levels of anxiety when faced with unexpected changes.
"A clear plan for informing claimants of likely delays would be a positive first step but more needs to be done. If the DWP wants to deliver a fair benefits system, it must suspend reassessments until the backlog is cleared."
Duleep Allirajah, head of policy at Macmillan Cancer Support, said: "Since the introduction of the new welfare system we know that far too many terminally ill cancer patients have waited weeks and months before receiving their much-needed benefits. Under the old system they only waited a maximum of 10 days.
"We are really pleased that the Government has listened to and acted on our concerns and we are now seeing waiting times coming down. But we want the Government to commit to processing all claims in under 10 days and look forward to seeing the publication of quarterly clearance times."
Shadow minister for disabled people Kate Green said: "This report confirms a continuing pattern of incompetence and failure under David Cameron.
"It's disgraceful that people with only weeks to live are facing such huge hold-ups.
"We've already seen the roll-out of Universal Credit become an unmitigated disaster. David Cameron must get a grip before this scheme descends into chaos too."