Ministers are under fresh pressure to pause the roll-out of Universal Credit after new figures showed around one in four new claimants wait longer than six weeks to be paid.
Labour said the system was in "total disarray", while Tory MP Heidi Allen also said the Government "should slow down a little bit and get it right".
Work and Pensions Secretary David Gauke said Universal Credit (UC) offered an unprecedented level of personalised support and he was committed to ensuring payments were made on time.
New data published by the Department for Work and Pensions (DWP) shows 77% of new claimants are paid their full benefit within six weeks.
This comes ahead of UC being rolled out to a much wider area - prompting warnings the system is not ready.
Shadow work and pensions secretary Debbie Abrahams said: "The Government's flagship Universal Credit programme is in total disarray.
"It is unacceptable that one in four claimants are waiting more than six weeks to receive support, alongside mounting debts and arrears among recipients.
"I have written to the Secretary of State requesting that he immediately halt the roll-out of Universal Credit to contain the misery being caused by the disastrous mishandling of this programme."
Mrs Allen, meanwhile, told the BBC: "There are a few things that are difficult, to get people behaviourally to change and also this system implementation issue, which for me just mean we need to slow down a little bit and get it right.
"We are tantalisingly close to an incredibly revolutionary system change, and I just want us to get it right."
UC combines a number of benefits such as housing benefit and tax credits into a single payment.
But research by the charity Citizens Advice found many using the system were waiting too long for their payments and getting into debts.
DWP data shows that 42% of families in arrears under UC say it is due to the waiting time to receive payment, support being delayed or stopped, or administrative errors in the system.
Some 47% of new claimants are receiving DWP loans to cover the six-week wait for their first payment.
Citizens Advice said that in August, the equivalent of 12% of people applying for UC turned to the charity for support.
Chief executive Gillian Guy said: "These figures confirm Citizens Advice research showing that Universal Credit risks pushing people further into serious debt.
"The DWP's own evidence shows more than one in five people applying for Universal Credit are waiting over six weeks for their first payment, and that many people say they are falling behind on their rent as a result.
"It is clearer than ever that the Government must pause the roll-out of Universal Credit and fix the problems with this benefit."
A panel of council leaders and welfare experts told the Work and Pensions Select Committee this week that UC Claimants "drop off a cliff" and "remain in freefall" in rent arrears due to delays in receiving payments.
DWP said that UC claimants are four percentage points more likely to be in work within six months than people who claimed Jobseeker's Allowance.
They argue the system mirrors the world of work, with monthly payments reflecting the way many working people are paid.
Mr Gauke said: "Universal Credit lies at the heart of welfare reform which is helping people to improve their lives by supporting them into work.
"Universal Credit provides claimants with an unprecedented level of personalised support which takes into account their individual circumstances and makes work pay.
"We have been rolling out Universal Credit in a careful, safe and controlled way, allowing us to make improvements as we go.
"We want to ensure that payments are made on time and that people can take up all the extra support that didn't exist under the previous system."