Benefit errors and delays are making families go hungry and putting their homes at risk, an influential Commons committee has warned.
The cross-party Work and Pensions Select Committee found that mistakes could leave "vulnerable people in desperate situations".
The MPs acknowledged that "many parts of the welfare system work well" but they found evidence of "too many errors and too many delays".
Committee chairman Frank Field said: "Delays and errors in delivering benefits are not just an administrative issue. Late or insufficient payments are plunging families into hunger, or putting their homes at risk."
The MPs said they were not able to fully scrutinise some benefit delays due to an "unacceptable" lack of information about Employment and Support Allowance (ESA) work capability assessments and mandatory reconsideration clearance times.
Mr Field said: "The paucity of data about benefit delays has made our scrutiny of this issue difficult, but as MPs we see every week in our surgeries the real hardship that is caused."
The report said the Department for Work and Pensions (DWP) should publish the data if they have them and "if they do not, then they are making policy decisions in the dark".
The MPs noted that the DWP had a target for reducing benefit overpayments, but not for underpayments which amounted to £1.5 billion in 2014-15.
"This should be remedied immediately as underpayments can have an enormous impact on claimants and their ability to pay for essentials," the MPs said.
The committee highlighted evidence submitted by groups working with vulnerable people who had been affected by benefits errors.
"Shelter told us that 'benefit problems often led claimants to face difficult decisions over whether to pay their rent or provide essentials such as food, gas and electricity for their household'.
"Citizens Advice reported that errors can push those less able to support themselves towards expensive borrowing and had 'clear consequences for physical and mental health, with stress and anxiety creating or exacerbating existing health conditions'.
"Others found that individuals can become reliant on food banks as a result of underpaid benefits."
The DWP expects its flagship welfare programme, Universal Credit, to simplify the benefit system and make it less open to mistakes.
But the MPs warned that the "legacy" benefits it replaces should not be neglected as the repeatedly-delayed UC will not be fully introduced for a number of years.
They added that there were concerns over the "five to six week wait "claimants have for their first UC payment.
"We are concerned the DWP has not properly considered households who have no savings or a final pay cheque to fall back on," the report said.
Labour MP Mr Field said: "The Government is betting the farm on Universal Credit, but that will not be fully implemented for several years and has already been subject to repeated delays.
"The department must not neglect the existing system in the hope that Universal Credit will save the day. On the contrary, it must do more to improve delivery now."
Tory committee member John Glen said: "Benefits must be delivered in a way that allows claimants to budget effectively. And for the most vulnerable claimants, timely and accurate payments are vital, across the full range of benefits.
"Universal Credit will be a welcome reform to improve benefit delivery, but whilst it is being rolled out, we must have the data to allow us to hold the DWP to account and suggest where improvements can be made."
Shadow employment minister Emily Thornberry said: "This week, thousands of families will be turning to food banks for their Christmas lunches. This failure of our social safety net is a national disgrace.
"This isn't happening just because people are having their benefits cut, significant as that is. All too often it's because they aren't even getting the support that they're entitled to in the first place.
"Instead of fixing these problems, the Government is making them worse. By introducing a six-week waiting period between applying for Universal Credit and getting the first payment, the Tories are establishing benefit delays, and the hardship they cause, as a permanent feature of the system.
"Alarm bells should already be ringing in the Department for Work and Pensions, and this timely report means that there won't be any excuse for ministers to ignore them."
A DWP spokesman said: "The reality is that the vast majority of benefits are paid on time and we have made huge improvements to the service we provide, meaning that benefits are now paid faster than ever before. It is disappointing that the committee failed to recognise this.
"Universal Credit, which will be in all jobcentres by April, is revolutionising the benefits system. It simplifies the system by combining six benefits into one, adjusts automatically as people's circumstances change, and ensures people are better off in work."